Both PLD and WAR have their specific abilities to generate hate and aggro, emnity, whatever.

If played to their highest strengths, aggro would be passed back and forth. In a coordinated group, aggro IS passed back and forth because it is necessary, and PLD can hold hate against a WAR.

In a non coordinated group, or when someone decides he has a bigger virtual sack then the other, neither the PLD nor the WAR can keep and hold the hate.

Abigs above mentions how a pally would have to be stupid to compete on aggro and gives his solution as to how a WAR keeps the hate. But a PLD, with his emnity amplifier on Shield Oath, can start his or her RoH combo, and before the last hit, pop provoke, finish the combo, start the next one, throw in Spirits, finish the RoH, and keep it. Oh wait! WAR has hate again because HIS combo finished and he used a cool down, so I'll wait for provoke.......

Coordinated, PLD and WAR do very well. Not coordinated, don't bother with Ultima (HM) or beyond.