In D&D 5e, spellcasters often have to choose between spells which can affect a target on a successful melee attack. The fire bolt cantrip (1d10 to 4d10 damage depending on character level), for example, requires an attack roll, while the poison spray cantrip (1d12 to 4d12) requires a Constitution saving throw by the target. Obviously, this could apply class features other than spells that require a saving throw to avoid an effect.

Assume two hypothetical spells which are the same level, cause the same damage of the same type, affect a single (and can thus be most directly compared with a single attack roll). Assume no resistances or immunities for the target, and assume the character is facing a creature of a CR matching their level (as they often would be as part of a party of 4-6). Also, assume that players are experienced, but are not allowed to consult the Monster Manual or other DM resources.

Possible considerations:

  • Many experienced players will know the AC of common monsters, but even when they don't, it's easy to figure out after a few attack rolls.
  • Most players won't often have knowledge of full monster ability scores, which determine saving throws.
  • Many DMs roll in secret. Even if they roll adversary saving throws in the open, the fact that there are 6 potential saving throws (and only 3 common ones) will make it hard in any given combat for a player to determine a creature's saving throw bonus.
  • Monsters have guidelines for ACs that are typical for a CR ("Monster Statistics by Challenge Rating", DMG p. 274).
  • Many monsters do not have any saving throw proficiencies. The DMG recommends that saving throw proficiencies primarily be used to counteract saving throw penalties from low ability scores.

Given this, and other considerations provided in the answer, from a player's point of view will a character succeed more consistently with an attack roll or a saving throw, or are the results of attack rolls more predictable than saving throws?

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    \$\begingroup\$ If by "consistency" you mean "% chance of success", then ultimately this question is going to come down to "does the monster have better AC or save" and "how easy is it to get advantage on attack / impose disadvantage on saves". Are you asking for a general overview of whether monsters typically have better saves or AC and how easily advantage can be triggered? \$\endgroup\$ – user-024673 Mar 18 '20 at 4:00
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    \$\begingroup\$ What are you trying to do with these results (are they theorycraft only)? \$\endgroup\$ – Akixkisu Mar 18 '20 at 8:26
  • \$\begingroup\$ This is an intriguing line of inquiry that leaves me wondering what problem you are trying to solve? Are you trying to get all d20 rolls to be based off of one basis? Hypotheticals tend to be better addressed in forum discussions (though what Axoren provided was certainly noteworthy) \$\endgroup\$ – KorvinStarmast Mar 18 '20 at 22:10
  • \$\begingroup\$ It is mostly curiosity. Partially I was thinking as player about how to weigh the likelihood of success of a alternative actions. But @TheLittlePeace suggests in a comment to a proposed answer that it could be useful information for homebrew spell design. \$\endgroup\$ – RedGeomancer Mar 19 '20 at 2:08

I have actually done analysis on this in the past and the answer is:

About 63/37 on AC vs. Saves, and it really depends on what you expect to see

The way I determined this was that I established equivalent levels of AC and Saves which would have the same rate of defense against a spell cast by the same caster.

You as a caster have a spell attack bonus of \$\text{Proficiency} + \text{Casting Stat}\$.
You as a caster have a spell DC of \$8 + \text{Proficiency} + \text{Casting Stat}\$.

So, we can deduct the variables from the situation and reduce the problem across casters.

Assume an attack bonus of \$+0\$ and a DC of \$8\$.

A spell with a DC of \$8\$ will be failed \$7\$ times out of \$20\$ with a save bonus of \$+0\$.
For a spell with an attack bonus of \$+0\$ to hit \$7\$ times out of \$20\$, the defender needs an AC of \$14\$.

This gives us an answer on a per-defender basis:

For defenders with an AC of over \$14\$ more than their save bonus, attacking Saves is easier.

Including most content released at least over a month ago, the splits across all published creatures seems to be as follows. I considered only the following defenses due to their common occurrence in spell effects: AC, CON, DEX, and WIS.

  • 503 creatures have AC as their weakest defense
  • 22 creatures have CON as their weakest defense
  • 96 creatures have DEX as their weakest defense
  • 178 creatures have WIS as their weakest defense

In case of ties, preference was decided as follows: AC > DEX > CON > WIS.

Some weakest defenses by creature type

  • Beasts 104/119 are weakest to attacks on AC.
  • Constructs 6/36 are weakest to attacks on AC.
    • 21/36 are weakest to WIS saves, but are immune to many spells which use those saves.
  • Dragons 39/47 are weakest to attacks on AC.
  • Monstrosities 68/96 are weakest to attacks on AC.
  • Ooze 8/9 are weakest to attacks on AC, Elder Oblex being weakest to WIS saves by a small margin.
  • Plants 21/28 are weakest to attacks on AC.

This section of the answer could do with a better presentation.

The following sources were used in this analysis:

  • Curse of Strahd
  • Dungeon Master's Guide
  • Hoard of the Dragon Queen
  • Monster Manual
  • Mordenkainen's Tome of Foes
  • Out of the Abyss
  • Princes of the Apocalypse
  • Rise of Tiamat
  • Storm King's Thunder
  • Tales from the Yawning Portal
  • The Tortle Package
  • Tomb of Annihilation
  • Volo's Guide to Monsters
  • Xanathar's Guide to Everything

Note: I use the term "weakest defense" fairly loosely here, but to reiterate, this is when considering only AC, DEX, CON, and WIS.

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    \$\begingroup\$ Solid analysis. I'm not exactly clear on what "weakest defense" means. I take it that it's AC or save that has the highest chance of being hit? Eg for AC 14, saves +1 it's AC, and for AC 15, saves +1, con save +0 it's con save? Is that about right? You conclude with "63/37 AC vs saves", does that mean AC is about twice as likely to be the weakest stat compared to everything else? Perhaps a discussion on advantage/disadvantage would help too. I believe advantage on attacks is very easy to get, where as disadvantage on saves is more difficult. This may further influence odds. \$\endgroup\$ – user-024673 Mar 18 '20 at 6:01
  • \$\begingroup\$ @pllpnakjlx Yes, essentially what it means is that regardless of who rolls the d20, the chances of the defender being affected is highest when the effect targets the weakest defense. The statement of being about 63/37 odds is a priori, meaning that it is a probability given absolutely no information about what you would likely be fighting in an adventure. Example: if you fight nothing but Beasts until level 20, then you're better off ignoring saves as a defense. Advantage and disadvantage on average are about +3/-3 to d20 rolls, allowing the \$\text{AC} - 14\$ rule to still apply on average. \$\endgroup\$ – Axoren Mar 18 '20 at 12:26
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    \$\begingroup\$ @Isaac Yes. If (AC - Save Bonus) > 14, then targeting a saving throw is more likely to affect the defender. \$\endgroup\$ – Axoren Mar 18 '20 at 14:51
  • \$\begingroup\$ I hadn't understood that sentence till your explanation just then. Possibly I wasn't thinking clearly, but the comment helped! \$\endgroup\$ – Isaac Mar 19 '20 at 20:19

For cantrips, they are equally predictable in damage output

Ignoring secondary effects and concentrating only on damage, the expected damage from a cantrip that uses an attack roll and allowing for critical hits is given in equation \$\eqref{eq1}\$ and for a saving throw cantrip in equation \$\eqref{eq2}\$. Note: these are strictly speaking only correct for 'middle' values where the chance of success is between 0.05 and 0.95 but that covers the vast majority of cases - if you are always (or never) going to succeed then it's not really a fair fight.

\$\begin{align} E(A)&={{D_a + (D_a+d_a)(21-AC+b+b_a)}\over 20}\tag{1}\label{eq1}\\ E(S)&={{(D_s+d_s)(7-s+b+b_s)}\over 20}\tag{2}\label{eq2}\\ \text{where}\\ D_a &= \text{Random variable representing dice from attack cantrip}\\ D_s &= \text{Random variable representing dice from from saving throw cantrip}\\ d_a &= \text{fixed damage bonus from attack cantrip}\\ d_s &= \text{fixed damage bonus from from saving throw cantrip}\\ b &= \text{spellcasting attack modifier = proficiency + ability modifier}\\ b_a &= \text{any bonus to hit with attack cantrip (e.g. *Wand of the War Mage*)}\\ b_s &= \text{any bonus to DC with save cantrip (AFAIK nothing does this but I don't know everything)}\\ AC &= \text{target's Armour Class}\\ s &= \text{target's saving throw bonus}\\ \end{align}\$

It follows that the saving throw cantrip is better when:

\$\begin{align} {{(D_s+d_s)(7-s+b+b_s)}\over 20}&>{{D_a + (D_a+d_a)(21-AC+b+b_a)}\over 20}\tag{3}\label{eq3}\\ {D_s(7-s+b+b_s)+d_s(7-s+b+b_s)}&>{D_a(22-AC+b+b_a) + d_a(21-AC+b+b_a)}\tag{4}\label{eq4}\\ \end{align}\$

Simple, right?

Well, let's see if we can make some assumptions.

  • \$D_a=\$ nd8 or nd10 for all Player's Handbook attack cantrips.
  • \$D_s=\$ nd12 for Poison Spray, nd8 for Sacred Flame, nd6 for Thorn Whip and nd4 for Vicious Mockery. Acid Splash we'll ignore because it can have multiple targets.
  • The \$n\$ in \$D_a=\$ or \$D_s=\$ is the same and in equation \$\eqref{eq5}\$ every term is a multiple of at least one of these so the level at which the cantrip is cast doesn't influence which is better, although it does influence how much better.
  • \$d_a=0\$ for everybody except a Warlock with the Agonizing Blast invocation using Eldritch Blast. We'll ignore them.
  • AFAIK there is no feature or magic item that makes \$d_s\ne 0\$ so we can let \$d_s= 0\$. Similarly for \$b_s\$.
  • There are things that can set \$b_a\ne 0\$ but they are pretty niche so we'll ignore them.
  • \$b\$ is in the range of 5 to 11 for most spellcasters.

So adopting these assumptions we get:

\$\begin{align} {D_s(7-s+b)}&>{D_a(22-AC+b)}\tag{5}\label{eq5}\\ \end{align}\$

This is more like it! However, it's still a function of 5 variables, two of them random variables. However, given the ranges of the random variables we know that there will not be a situation where one is strictly better than the other so we can substitute the expected values and get some sensible insight. \$E(D_a)\$ is either 5.5 or 4.5 and \$E(D_a)\$ can be 6.5, 4.5, 3.5 or 2.5.

I'm not going to comprehensively compare cantrips but I will look at the one you start with Fire Bolt v Poison Spray and I'll assume a caster with \$b=5\$ (typical for a 1st level). The saving throw cantrip is better when \$s\$ is less than or equal to:

\$\begin{array}{c|c} AC & s\\ \hline 8 & -5\\ 9 & -4\\ 10 & -3\\ 11 & -2\\ 12 & -1\\ 13 & 0\\ 14 & 1\\ 15 & 1\\ 16 & 2\\ 17 & 3\\ 18 & 4\\ 19 & 5\\ 20 & 6\\ \end{array}\$

So, it's almost a 1 for 1 exchange - high AC monstres are more likely to be better targeted by a saving throw spell. Which is not really a deep insight for all that maths but now you know that the "normal" range of AC and "normal" range of save bonuses overlap, more or less so there really isn't much to choose between these cantrips.

At the other end of the scale Vicious Mockery is never better than Fire Bolt but then, you shouldn't be using it for it's direct damage output anyway.


The question wrongly assumes landing an attack is just as important as failing a saving throw

In reality this is almost never the case as many saving throw spells can deal half damage even on a successful save whereas attack rolls will entirely miss. Likewise, many saving throw spells will have more lasting effects, often involving even more saving throws which is very rarely the case with attack rolls. On top of this nearly ever attack roll will be single target and many saving throws will be multi-target. There is no reason to claim these methods of spell delivery have an even playing field. They have different effects, benefit from different features and abilities, and target very different aspects of a given enemy.

Even taking the assumption for granted, it requires nothing more than a basic statistical analysis of every single monster

For the reasons already explained, comparing the two is a pointless endeavor, but let's humor the hypothetical. It would suffice to check every single monster of each CR and see which options are best to use and worst to use and by how much. This is a rather horribly lengthy endeavor but requires little more than calculating the average AC, Str, Dex, Con, Wis, Int, and Cha save bonuses of every monster of each CR. And so, it would be immediately apparent which option is "best" on average against the CR-appropriate monster.

Any such analysis will be unusable/irrelevant because the assumption is false

You cannot simply learn that, for example, AC is on average the best thing to target, and then go about choosing only spells that involve attack rolls, or even mostly spells that involve attack rolls.

So many options and effects will be completely missed out on simply because of what was statistically "best". Even if you looked at the average scores are chose a number of spells of each method to match the percent of creatures where that is their weakest defense you would still have problems.

This entire analysis would depend directly on the premise that all attack rolls spells and all saving throw spells are entirely and exactly equal in their damage and effects. This simply cannot be the case or we would not even have the massive variety of spells that we do. No meaningful information or data can be extrapolated from this assumption.

Additionally, as MarkWells pointed out, such an analysis would assume an equal likelihood of encountering every single monster (including NPCs existent in a single module) as well, which is never the case.

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    \$\begingroup\$ @Red I just find the assumption you're asking answers to make to be utterly absurd and entirely useless. No meaningful (actually applicable to play) answers or data can be obtained from it \$\endgroup\$ – Medix2 Mar 18 '20 at 18:18
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    \$\begingroup\$ There's the related problem that you can't aggregate across monster types without knowing the likelihood of encountering each of them. This is not a theoretical problem--if you naively include all monster statblocks in WOTC-published material, a ton of them are unique NPCs from particular adventures, and including them in your analysis will produce garbage results. \$\endgroup\$ – Mark Wells Mar 18 '20 at 20:26
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    \$\begingroup\$ @TheLittlePeace If you're homebrewing a spell and need to know whether it should use an attack roll or a saving throw, you consider how the spell works conceptually. Could it be deflected by armor? Then it's an attack. Could it be dodged? Dex save. Is it physical trauma that can't be dodged? Str save. Can it be overcome through force of will? Cha save. And so on. This way, players know what to expect. \$\endgroup\$ – Mark Wells Mar 18 '20 at 21:32
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    \$\begingroup\$ The question gives an example where it's meaningful. Which cantrip is a more reliable way to damage enemies, fire bolt or toll the dead? Low level spellcasters don't have the luxury of picking both and using whichever is more appropriate for the situation. \$\endgroup\$ – Miniman Mar 18 '20 at 21:35
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    \$\begingroup\$ All of which is fine, but as a 1st level wizard with only one cantrip to spare for damage, you have to make a choice. Damage type is one factor that informs that choice, and we have a corresponding question about which damage type is best. Saving throw vs attack roll is another factor that informs that choice, and it's perfectly reasonable to address that factor. \$\endgroup\$ – Miniman Mar 18 '20 at 23:17

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