Ahh, the famous "I don't have a DPS stance yet, so I should MT" Lv50 Warrior quote.

Judging all three tanks really quickly for Lv50 content:

Warrior
--------
MT WAR explains itself. While no DPS stance, you have a skill to off-set the damage penalty of your stance. Unchained + Defiance, build 100 beast gauge, dump 100 gauge + infuriate on 3x Inner Beast (ideally in Berserk popped right after you have the full gauge), then turn the stance off and play Warrior like a semi-dps until next Unchained is up.

No-stance + Eye + Berserk OT WAR is stronger than DPS Paladin though.

Paladin
--------
The difference between WAR and PLD at 50? WAR has no DPS stance, PLD has no DPS combos. Your stance dictates what you are doing, but neither is really a difference. Should MT over DRK in alliance raids (cuz no Shirk there) unless DRK goes heavy on Grit for a while.

Dark Knight
-------------
Good flex-pick, can do well on both MT and OT. Should OT tho if someone else WANTS to MT.