Hm, how to do this..
Prot paladins have more PvP utility than ret paladins. They have the only silence and slow available to the class, the only talent that buffs Divine Shield, the largest damage reduction and stamina increase talents, and the best AoE damage.
They have Blessing of Sanctuary and undispellable Divine Plea, but with their tiny mana pool, they need someone attacking them to really keep their mana up. The lack of Divine Purpose and Repentance makes them even more easily kitable than Ret, especially with the tiny mana pool preventing Cleanse spam.
Ret paladins have more single-target DPS and better burst. They have MUCH better mana regen, especially when attacking casters and healers. They can remove stuns from themselves or party members on a reasonably short CD; one that also doesn't apply Forbearance. And they have Art of War. Instant-cast self heals are extremely useful in PvP.
Ret paladins lack the Shield of the Templar silence, and are stuck with Hammer of Justice and Repentance as their only ways to interrupt casters. Seal of Justice is too random and too large a DPS decrease. Killing a healer is tough as either spec, though.
Either do well in BGs. I don't think the difference between them is very large, though, so I suggest you try it both ways. I'd say there's far more difference between Prot and Arms warrior than Prot and Ret paladin.