I picked up a priest - and for raiding was Disc / holy, since some fights are more AOE based and others I would MT heal or something. Your sick in arena's, and will be more offensive in cataclysm then we are now.

You can go heals / dps, which alot of raids and guilds want constantly to fill roles. There are always more spots for healers then there are tanks, since almost all raids are balanced around 3 tanks 5-6 healers and rest dps.

Paladin though, your great in arena too... you can do melee dps and tank, but I don't think they are as diverse as a priest.