Like I said in another post... I judge things the same way The Bureau does: I run the numbers.
Which is worse? A bad player with a good spec or a bad player with a bad spec? Which is better? A good player with a bad spec or a good player with a good spec? Which is better still? Class that has gear that actually works for it? Or a class that Blizzard, after designing the God damned game for three years, has no idea how to make gear for themselves?
Even if you take things as subjective as player skill into the equation, the bottom line is you are still likely to see far worse Ret Pallies than you are any other DPS class.
EDIT: Even if, for argument's sake, the OP could turn out a perfectly acceptable Ret Pally, he did ask which would be better. And objectively, I don't think even you Ret Pallies would argue that, unless he's got some specific attatchement to the Paladin class, a Shadow Priest would be better. Most end-game guilds won't touch Ret Pallies with 39 and 1/2 inch pole, let alone invite them to heroics, raids, or PvP teams. Shadow Priests, on the other hand, are welcome in all three.
Plan to play Retribution if you want to play a Paladin first, and pick your spec second. If you are objectively trying to achieve a desired result--in this case, a class that can switch back and forth between a healing spec and a solid DPS spec--go with Priest.
Edited, Jan 15th 2008 6:44pm by Gaudion