WARs have two resources for big pulls (TP and MP). PLD only have one.
WARs run out of TP fast on big pulls. PLD run out of TP more on bosses.
WAR have no way of recovering resources. PLD have Riot Blade combo to recover MP.
WAR can mitigate big hits better. PLD can mitigate lots of smaller hits better.
WAR have a unique debuff (Storm's Path). PLD have a unique debuff (Rage of Halone).
WAR tank stance is not on GCD but you lose HP if you drop it. PLD tank stance is on GCD and you lose MP if you switch.
WAR can shut off tank stance damage reduction (Unchained). PLD can't shut off tank stance damage reduction.
WAR have an off GCD stun to easily stop specific abilities. PLD have a GCD stun to easily stun lock certain mobs.
WAR cannot silence a mob. PLD can silence a mob.
WAR cannot cover an ally. PLD can cover an ally.
WAR cannot Stoneskin self or others during downtime. PLD can Stoneskin self or others during downtime.
WAR can pull enemies closer and bind them (Holmgang). PLD can break heavy and bind and prevent knockback (Tempered Will).
WAR emergency button (Holmgang) saves you but you will need major healing. PLD emergency button (Hallowed Ground) saves you and you won't even take damage.

Both tanks are very good. Some argue WAR is more fun because you are not spamming 1-2-3. However, your rotation really is not much more complex: occasional Inner Beast, refresh Maim with Storm's Eye or Storm's Path combo. PLD has more general utility (back when silencing ADS was a thing, for example) and you have more skills to directly aid the team (Cover and Stoneskin).

I'm not an endgame expert but I play both tanks all the time and both have fights they are better suited for. In fights that require 2 tanks, I think I'd almost always bring 1 of each.