Arcane Mastery vs Spell Penetration which is better to consistently overcome SR?

Question

Spell penetration gives a +2 to overcome spell resistance.

Arcane Mastery allows you to take 10 on a caster level check to overcome spell resistance.

You have a 55% chance of getting a 10 or higher when rolling a d20.

And most monster SR's are fairly low... which feat will more consistently land?

Answer 1

What kind of campaign is it?

A monster's spell resistance typically equals its challenge rating +5 or, more often, +10. According to Table 3–2: Encounter Difficulty (Dungeon Master's Guide 49), most encounters (80%) will have an encounter level (EL) equal to or less than the four PCs' level. (The table assumes four PCs, all the same level.) With this in mind, in a typical by-the-book campaign for a caster whose caster level equals his effective character level, the feat Arcane Mastery (Complete Arcane 73) seems to this a player a better—but still not a great—choice: with already a 55% chance to best the spell resistance of at least 80% creatures encountered, taking 10 on the caster level check to overcome spell resistance means overcoming spell resistance, on average, all the time.

However, in a nonstandard campaign—as I suspect most are this late in the game's lifecycle—there's no way to know what the DM will throw at the PCs. An experienced DM knows when a lower-than-normal EL encounter is a deathtrap and when a higher-than-normal EL encounter is a walk in the park, despite the DMG's guidelines. Further, there's no way to anticipate what kind of challenges will be encountered when playing with a DM who makes the campaign Nintendo hard. In such cases, a better bet is the feat Spell Penetration (Player's Handbook 100), if only because, to overcome the tailored or optimized challenges in such DMs' campaigns, the PCs will need all the bonuses they can get. Taking the median isn't going to help.

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Note: This reader recommends neither the feat Arcane Mastery nor the feat Spell Penetration for most casters; they should, instead, be finding ways to increase their caster levels and using spells with entries of Spell Resistance: No against creatures they identify as possessing huge amounts of spell resistance (or running away… or employing the Spell Compendium spell assay spell resistance (17) as this answer suggests). This reader does recommend the feat Arcane Mastery to casters (and especially warlocks) who have ready access to a number of dispel magic effects as being able to take 10 on caster level checks to dispel magical effects—rather than to overcome spell resistance—is certainly and frequently a thing.

Answer 2

\$\DeclareMathOperator{\vs}{vs.}\$Here is a table of how much better (the ratio of the success rate with the feat to the success rate without it, expressed as a percentage—100% means the feat does not change your success rate) each feat is. \$SR - CL\$ is the target spell resistance minus your caster level (before either feat), that is, the roll you need on the d20 to succeed.

SR − CL	Arcane Mastery	Spell Penetration
≥23	N/A¹	N/A²
22	N/A¹	∞ (10% vs. 0%)
21	N/A¹	∞ (5% vs. 0%)
20	N/A¹	300%
19	N/A¹	200%
18	N/A¹	167%
17	N/A¹	150%
16	N/A¹	140%
15	N/A¹	133%
14	N/A¹	128%
13	N/A¹	125%
12	N/A¹	122%
11	N/A¹	120%
10	200%*	118%
9	181%*	117%
8	154%*	115%
7	143%*	114%
6	133%*	113%
5	125%*	113%
4	118%*	112%
3	111%*	111%*
2	105%*	105%*
≤1	100%†	100%†

1. _{For SR − CL greater than 10, you cannot use Arcane Mastery (or, doing so guarantees failure), so it has no effect on your success rate.}

2. _{For SR − CL greater than or equal to 23, you cannot succeed even with Spell Penetration, so the feat has no effect on your success rate.}

* _{Guaranteed success with the feat.}

† _{Guaranteed success even without the feat.}

So Spell Penetration helps in a lot more situations, and helps more in its best use-cases. But Arcane Mastery guarantees success against far more creatures, and creatures with SR high enough to really put Spell Penetration to good use, are really creatures you really don’t want to be checking spell resistance against. Even though Spell Penetration literally gives an infinitely-greater chance of success if the SR was 21 or 22 higher than your CL, you are still left with only a 10% or 5% chance of actually beating the SR with Spell Penetration. Something with SR that high is far too dangerous to take odds like that—you need to use SR: No spells, or probably better, running away.

Which is really the big problem with either approach to SR: you are investing a huge amount (a feat) into beating SR, when you really want to not have to. There are a lot of alternatives for spellcasters to avoid having to actually roll against SR at all, and they are often very good.

And if you really want to use an SR: Yes spell in such a situation, these feats aren’t the best answer anyway. That would be assay spell resistance from Spell Compendium—that applies a whopping −10 penalty to a target’s SR. If you have SR: Yes spells that you want to be able to use, prepare a few copies of assay spell resistance, and be done with it.

All that said, Arcane Mastery isn’t a bad feat for other purposes, perhaps most notably dispel magic. If you must pick one of them, pick that one just because it does more than just SR.