I've played a priest healer as a raid character since ZG and well... I vote priest!
Also go Disc spec. Holy isn't that good anymore in my opinion. A full disc spec'd priest with plenty of spellpower and crit is in my opinion the strongest single target healer available and has the option to sit back and spam AOE heals if stuff goes bad. Also PoM is awesome for a non pally tank because it causes AoE threat for the tank. makes multi tanking on my druid much easier.'

Edit: Oh and also priests healing/shields are proactive instead of reactive allowing you to technically give whoever your healing the largest effective health pool compared to any other healer.