WAR: 11250 HP (in Defiance)
PLD: 9000 HP (in Shield Oath)

Tank buster: 8000 damage (completely unmitigated)

WAR and PLD both don't pop any CDs.

WAR: 11250 - 8000 = 3250 HP remaining
PLD: 9000 HP - 6400 (Shield Oath is -20% damage taken) = 2600

Auto attack: 3250 damage (completely unmitigated)

WAR: 3250 - 3250 = dead
PLD: 2600 - 2600 = dead

WAR won't die any faster than PLD, PLD won't die any faster than WAR. WAR takes like 1% more healing to actually heal up, though - which is pretty much negligible.