Shadow Wall vs Vengeance!

[Marveth]

I can count Grit because it gives damage reduction. The only mitigation-related part of Defiance is Parry. If you want me to include all the HP stuff as well, then I'd have to do so far DRK too, and again it comes out as very comparable.

There is a vast difference in healing and max health increased. Max health increased can be compared to mitigation due to the way that raids work in FFXIV. Yes I compare to raids because that is where the difference in skills between the classes will really show.
The way raids work in FFXIV currently is that bosses have tank buster hits that does massive damage which can 1hit KO or close to if you are not weary. Damage mitigation prevents this from happening, just as having more health over all can prevent you from dying to these devastating attacks. In between these big hits the tanks are not really that stressed and healers spend most of their times healing other people. So yes, skills that add to max HP has to be taken into consideration like Defiance and Thrill of Battle. Skills that only heals however, not as much.

[Faytte]

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Everything said

^ This is 100% accurate. I tanks for 2 years as a warrior (up to t13 pre echo). Basically if tank health at a particular phase is X (any number) then a tank buster will generally hit for X+30% so without some extra mitigation, you tend to die. A warrior increasing their health up 25% is effectively a layer of mitigation and works as effectively as grit/shield oath. I.E

if i have 10K health, and a 13K attack is coming in, then a paladin will take 10.4K damage, and a warrior will still take the 13K but would have had 12.5K health (basically, both are death by a similar margin, and if both pop 1 cd, they are suddenly NOT dead but a similar margin).

Edit: Simpler math is, pld/drk take 20% less damage against a pool of numbers, where warriors have 25% more in that pool of numbers. A value applied to the pool is more or less the same % of the pool.