DEX-focused heavy armour user - mithral full plate vs mithral breastplate

Question

So I'm curious about a matter of best AC for my buck. I'm playing a Dexterity focused heavy armour character (Cavalier to be specific, but I imagine the same answer applies to fighters or any other heavy armour proficient characters), My starting Dex was 16, so without taking into account a Belt of Incredible Dexterity my DEX by level 20 will max at a +5 bonus.

I'm trying to decide whether I am better off using my heavy armour proficiency, getting Mithral Full Plate and accepting that I can onlyuse +3 of my DEX bonus for AC - though I believe I can still get my full DEX bonus to other things such as Weapon Finesse and Deft Strike (when I multiclass into Swordlord later on) and Reflex saves.

Or am I better off with something lighterweight like Mithral BReastplate, even thought he base AC is lower, get to apply more of my DEX to my AC and not take the movement speed penalty for wearing Medium armour which I would for having Full Plate.

I'm not especially confident in my math as to which can get the better overall AC - as I am a front-line tank and really my ideal situation is to maximise AC over any other concerns.

Answer 1

AC from Dexterity vs. AC from armor

Your Dexterity bonus to AC is vastly superior to your armor bonus to AC, because your Dexterity bonus applies against touch attacks and your armor does not. That makes bonuses from Dex twice or more as valuable as bonuses from armor. Thus, even if the total AC ends up being lower, the mithral breastplate is probably better defensively—and that’s before we even get into things like movement speed and armor check penalties.

Mithral breastplate vs. even lighter armors

Honestly, it’s entirely likely that your best option is actually a mithral chain shirt or darkleaf lamellar leather, rather than a mithral breastplate. Those have even lower penalties and even higher max Dex. I realize that takes you away from the Dex+heavy armor theme you had going on there, but at higher levels a +5 Dexterity bonus is rather poor for a Dexterity-focused character–you are quite likely to exceed that by a considerable margin, and therefore may very well reach a point where the lighter armor would be better than the mithral breastplate. This does depend on how high in level you expect to get, how likely you think it is that you’ll be able to trade your (heavily enchanted) mithral breastplate for an equivalent light armor, and so on, but in many games my preference would be for a lighter armor.

About AC in general

This is a “trap” in Pathfinder (and its ancestor, D&D 3.5e) that a lot of new players fall in, because the game makes AC out to be your first concern for defense. AC is the worst defense in the game. You should not focus very many resources on it.

It’s often claimed that the “Big 6” magic items that every Pathfinder character should get include three separate AC-boosting items, that is, a ring of protection and an amulet of natural armor on top of magic armor. A +5 armor, a ring of protection +5, and an amulet of natural armor +5 costs a staggering 125,000 gp. That gold gets you +15 AC, +10 of which doesn’t apply against touch attacks. And after all that, your AC is likely still mediocre, at best, and you’ll still be hit by most attacks.

Meanwhile, attacks almost always deal hp damage, which means even if you are hit, you have some wiggle room before it really matters. If you fail a saving throw, you are—at best—out of the fight. At worst, you could be turned against your allies. That makes your saves vastly more important to increase. And 125,000 gp can get a lot of saving throw bonuses, a lot of heavy resistances or immunities, and a lot of alternative protections even against attacks, like miss chances. It’s vastly cheaper to get a 50% miss chance than it is to get AC high enough that high-level foes are going to miss 50% of the time.

So the correct (optimal) thing to do for most Pathfinder characters is to get a +1 armor, and then ignore AC from then on. Maybe a ring of protection, since that covers (far more dangerous) touch attacks, though it’s also very expensive so you probably don’t want to go beyond +1 or so. Other defenses are more effective and vastly less expensive.

Anyway, if you really do want to get high AC, a high-Dexterity build is mathematically your only hope. With high Dexterity, you still have to spend a ton of gold—gold that may very well be better spent elsewhere—but at least by the end of it you have respectable AC that may actually pose something of a problem for enemies.

Answer 2

Summary: There isn't a huge difference in performance between the options at higher levels, despite significant differences in costs.

╔═════════════════════╦═══════════╦══════════╦═════════╦═════╦═══════╦══════════╦══════════╗
║       Option        ║ Cost (gp) ║ AC Bonus ║ Max Dex ║ ACP ║ Speed ║ Total AC ║ Touch AC ║
╠═════════════════════╬═══════════╬══════════╬═════════╬═════╬═══════╬══════════╬══════════╣
║ Steel Breastplate   ║      2220 ║        9 ║       3 ║  -6 ║ 20'   ║       22 ║       13 ║
║ Mithral Breastplate ║      7220 ║        9 ║       5 ║  -1 ║ 30'   ║       24 ║       15 ║
║ Steel Fullplate     ║      3520 ║       12 ║       1 ║  -8 ║ 20'   ║       23 ║       11 ║
║ Mithral Fullplate   ║     13520 ║       12 ║       3 ║  -3 ║ 20'   ║       25 ║       13 ║
╚═════════════════════╩═══════════╩══════════╩═════════╩═════╩═══════╩══════════╩══════════╝
(Includes a Heavy Shield of matching material, and +1 enchantments)

╔═════════════════════╦══════════╦═════════════════╦════════════════════════════╗
║       Option        ║ Total AC ║ CR for 50% miss ║ Extra AC for 50% miss@CR20 ║
╠═════════════════════╬══════════╬═════════════════╬════════════════════════════╣
║ Steel Breastplate   ║       22 ║               6 ║                         19 ║
║ Mithral Breastplate ║       24 ║               7 ║                         17 ║
║ Steel Fullplate     ║       23 ║               6 ║                         18 ║
║ Mithral Fullplate   ║       25 ║               7 ║                         16 ║
╚═════════════════════╩══════════╩═════════════════╩════════════════════════════╝

Details: Focusing on the mathematical mechanics of the question, we need to examine the differences in properties between the options. Assuming a shield made of the same material, and a +1 enchantment on both armour and shield, the first table above compares the differences in total AC, Armour Check Penalty and Speed. The last two columns summarise the effective full and Touch ACs of the options. As you can see, there are only a point or two of difference between the options.

A higher Dex mod won't change these details, either, but might impact on other combat stats. Most other AC buffs would then stack the same on top of any of these options. If you were to end up with a higher dexterity, then lighter armour options might be worth considering. The Celestial Armour is an outstanding option for medium armour, with a Max Dex bonus of +8.

The overall differences are minimal between the options, for a considerable difference in cost. The major difference is in the base movement speed, which may or may not be an issue, depending on if you are mounted, and how often you have buff spells like Haste or Fly available.

For comparison, you can look at the expected monster stats by CR, and you will see that even the Low Attack option will be hitting AC25 at least 50% of the time by CR11. By CR20, you would need at least another 16 points of AC to give the high attacks a 50% chance to miss. Even the lowest level creatures will be hitting the Touch AC at least 50% of the time, and almost always by CR11 at the latest. The second table shows how much extra AC bonus you would need to add to each option at level 20 to have a 50% chance of attacks missing.

Also note that an Agile Breastplate is generally a slightly better option over the Breastplate, as this reduces the armour penalties for Climbing and Jumping a little further.