WAR and PLD are essentially equal, but there's some situations in which you may want to use a WAR over a PLD, and vice versa.

For T6 and T7 we use one of each as there's no point in having two of the same, especially for LB building.

In T8 we use a single WAR tank. More damage = we push phases faster and the fight ends faster. WAR also has great snap threat for those Dreadnaughts.

In T9 we use a single PLD tank. PLD has way more cooldowns than WAR which can be spread out, which is essential to surviving Ravensbeak/Blight and Bahamut's Claw.

The changes in 2.1 really closed the gap between the two, so it's not like there's a clear cut winner between the two in every piece of content anymore.