So I've decided to make the jump into WoW. I played SWTOR for a while and my favorite character was a Sage. I also played a lot of City of Heroes and my favorite character was a Defender or Controller.
I've always enjoyed the ranged damage dealer. I thought about a Mage, but I tend to like a little more diversity (perhaps I'm wrong in my assumption here) with my characters.
As far as my play style. I like to quest but plan on doing dungeons as I level up and eventually raids at some point. I will jump into pvp from time to time but it won't be that often.
The only real advise that can be given really is to try them both out. I main a lock and I have a priest that I think is a blast to play. I started as a lock when I began playing so I will always have a soft spot for them. I made my priest to try one out and I liked it too. Like I said though, just try them both out and see what clicks with you.
If you want to heal, go Priest because Locks can't heal. If you don't mind not being able to heal, then Locks are better in like every way. Locks recently got a major revamp up from their 2005 mechanics, and they're definitely the most fun caster damage dealer at the moment.
One day dude, I'm just gonna get off the bus, and I'm gonna run in the woods and never come back, and when I come back I'm gonna be the knife master! -The Rev