Paladin is better at generally taking less damage through the course of the fight. Their cooldowns are more powerful, their Shield provides free mitigation, and Shield Oath works wonders.

Warrior is currently better at mitigating large, predictable hits. This is because the relatively low cooldown on Inner Beast allows a well timed Beast to reduce the damage of all "Tank Buster" hits in a fight by 20%. They also completely wreck face while Tanking due to abilities that negate the damage penalty of their Tanking Stance.

The whole Off Tank thing is one big "it depends".

Regarding Damage Dealt:
-If Slashing Resistance is on the boss, a Paladin will deal (slightly) more damage as Off Tank than a Warrior will.
-A Warrior will deal more damage as the Main Tank than a Paladin will. Thus, if damage is a worry you want Warrior MT and Paladin OT.

Other Considerations:
-Warriors are better at quickly picking up a group of adds.
-Both are competent at picking up a single new add.
-In a bind, an Off Tank Paladin can Cover the Warrior.
-Off Tanking allows a Warrior to easily maintain both Storm's Eye and Storm's Path on the enemy.

It goes without saying that both Tank can do whatever they put their minds to. They're pleasantly balanced like that. In the end, it'll come down two two things: whether or not raid DPS is an issue and personal preference.