. . . . . wut?
If by "prime tank" you are referring to "main tank" I'd have to say it is all due to preference, though in the most preferred situation, Paladin would be the main tank with Warrior as off tank. Reason being, Paladin are preemptive and Warriors are reflexive. Main tank and off tank are not based off of AOE power, but the ability to tank bosses better. Of course, it all comes down to player skill and preference, but in the ideal party makeup, you would find the Paladin on the boss with Warrior on adds due to the nature of the classes.