Looking at sims will give you an answer quite quickly - however, for single target DPS, Ret will pull ahead of spriests as they're designed around more than one target. Spriests will easily overtake a ret paladin against 2 targets.
Both are really, really low on dps. Ret aoe is 100% RNG based which can be bad or very good, for a raid spot Devotion Aura that rets bring can be very valuable but then there's the whole ranged vs melee.
Doesn't really matter what you play as long as you fit the raid you're in.