I have done this VERY quickly just before going to work so I almost certainly missed something. EDIT: I forgot Vengeance... so changing these calculations again as below.
Talking of tank busters, I decided to try and compare some hypothetical numbers. It doesn't matter if they are accurate in this case, as it's pure theory, as long as they start from the same premise.
If a DRK has 1000 health than the WAR would have 1250 with Defiance on. In here I presume that WAR has Defiance and the DRK has Grit.
WAR : 1250 health (Defiance up) - 800 hit = 450 (36% remaining life)
DRK : 1000 health - 640 hit (Grit up) = 360 (36% remaining life)
(Thanks Reynhart)
WAR Health 1250 +20% Thrill of Battle = 1500 - 800 hit (-80% Raw Intuition, Storm's Eye and IB, Vengeance) = 1340 + cure 300 (+25% Defiance) = 1715 (114% of health before Thrill of Battle wears off)
Total effective health output: 250 from Thrill of battle + 640 from mitigation + 375 from Cure with Defiance buff = 1265
PLD Health 1000 - 800 hit (- 70% Grit, Shadow Skin, Shadow Wall) = 760 + cure 300 = 1060 (106% of health - 60 overhealing)
Total effective health output: 560 from mitigation + 300 from Cure - 60 overhealing = 800 (860 if you count overhealing as health pools are larger than 1000)
(This was before EDIT: Therefore I do not think that WAR are WAY ahead of DRKs from a defensive point of view.) AFTER EDIT: WAR has about 32% more effective health when the conditions above are met.
Magic VS Physical Damage
Let's presume that there is a new tank buster in Heavensward, which is magical. Let's repeat the calculations above.
WAR Health 1250 +20% Thrill of Battle = 1500 - 800 hit (-60% Vengeance, Storm's Eye and IB) = 1020 + cure 300 (+25% Defiance) = 1395 (93% of health before Thrill of Battle wears off, some of which effectively becomes overhealing)
Total effective health output: 250 from Thrill of battle + 480 from mitigation + 375 from Cure with Defiance buff = 1105
PLD Health 1000 - 800 hit (- 85% Grit, Shadow Skin, Shadow Wall, Dark Mind) = 880 + cure 300 = 1180 (118% of health - 180 overhealing)
Total effective health output (including overhealing): 680 from mitigation + 300 from Cure = 800 (980 if you count overhealing)
But DRK has Delirium (-10% INT) and I don't know how much mitigation that is.
WAR is ahead of DRK even in tanking magical damage without counting Delirium, but Delirium will not make up for the gap, although it will bring them closer.
Extra Considerations
After the hit WAR could use Equilibrium (1200 potency) to heal (if the tankbuster happens less often than every 60 secs) VS DRK's use of several Souleaters (260 potency without DA, and 400 potency with DA) to heal.
IB is up way more often than Shadowskin or Shadow Wall, but DRK has 20% from Grit. So compare IB to Grit, DRK has the advantage.
Raw Intuition has a CD of 90 seconds SAME as Shadowskin and they both offer 20% damage reduction, but Shadowskin has no need for Awareness (I know RI is bugged, but I am not going to factor that in, I also know that with good position you can use it without Awareness, but it will prove clunky with a lot of mobs).
DRK has a 30% damage reduction every 180 secs (Shadow Wall), WAR has Vengeance on 150 secs CD.
DRKs -10% damage from Reprisal might be up sometimes, but as it's RNG, I didn't factor it in.
DRK's Dark Mind is a trap, since it's "extra magic damage" but actually, it isn't extra when you add it all up.
Dark Dance gives you an extra 30% parry chance, add it to the 20% parry chance, it means you have 50% parry chance. That is a 50% chance that the buster will be mitigated by 20% IF it's physical. Dark Dance is more effective against groups. You can give it extra evasion if you use Dark Arts. You can dodge spells (I dodged an this morning Ice Sprite), Dark Dance with DA makes sense only if you use it against magical damage.
Damage Considerations
These are not proper calculations just a "off the top of my head" thoughts.
Damage wise, WAR gets a DPS stance that gives them 5% extra damage and crit depending on stacks, they also get Maim (20% damage), Berserk and Inner Release. But they have attacks with considerably lower potency. They also get Fel Cleave and Decimate with 5 stacks of Abandon.
DRK keeps 15% from Dark Side, plus Blood Weapon (attack speed) attacks with considerably higher potency and enhanced Souleaters with DA, which with Blood Weapon can be executed quite often.
The WAR might pull slightly ahead here but I don't think it's trashing the DRK.