Pick the class you prefer to play is all i can say.
I myself play resto shaman and i love it. You have so many tools as a resto shaman.
Lots of totems for utility = raid hand of freedom, aoe root, aoe stun, mana tide, stormlash totem
And also alot of cd's since mop = Healing Tide Totem, Ascendance, Spirit Link Totem, even Fire/Earth elemental totem (10 % more healing for 1 min) if you spec into primal elementalist.
I don't know much about priest, so i will let someone else tell all the good things about priest healers
From a gearing POV i would say go priest though, since you will be the only cloth spirit user instead of 2 x spirit mail users
oh yes. i always forget about what a wide range of things hunters can actually do. in anycase, it would allow your hunter to respec to SV or MM if he wanted
(i beleive only BM brings the hero? could be wrong again though)
I would say whichever class has a DPS offspec you enjoy playing. It will be necessary on some heroic fights to drop down to two healers and it will help a lot if you have an offspec that you're good at and don't mind utilizing.