Being someone who's always been a healer I'll give you my 2 cents on your RBG healer choice :)
I picked up MoP rather late so I've only been 1800ish this season due to lagging gear, but the current state of healers in RBGs is something like...
Monks->strongest overall healers, best throughput, great survival, and getting buffs next patch(rofl every RBG group will want a MW come next patch, I promise you that)
Holy paladin->groups tend to prefer a pally with FC for freedoms and strong single target heals, sac, etc...you won't come near a monk's healing, but most groups run at least one holy pally. There's also a TON of holy pallies so IMO you're better off rolling one of the other classes
Resto shaman->I'd be lying if I said I know much about em. I know in arena they dominate but in RBGs I think they lack the capacity to put out the level of healing a monk/druid/h-priest can.
Resto druid->strong throughput, good survival with ToL CD, strong mobility
Holy priest->maybe better overall throughput than monks next season once holy priests get 30% blessed resilience and can actually survive. You have a lot of instant heals as a holy priest and some solid CDs to help the raid(glyphed lightwell, divine hymn, GS)
Disc->really strong single target heals. IMO much harder to heal an RBG with due to less instants than holy. CDs arent nearly as effective as holy's CDs.
So to sum it all up, I think most groups are going to want either a MW monk or holy priest, maybe both come 5.2. Both should out-heal all the other specs in a group situation and hopefully the 30% blessed resilience will be enough to keep h-priests alive. I notice you're considering disc, but for RBGs the many instants holy has are awesome. You don't tend to do much DPS as a healer in RBGs, but next patch holy priests will be able to swap chakras every 5 seconds and that means swapping to a very solid DPS chakra if you choose to.
And groups tend to go after priests and shamans first. That may change next patch when priests get the 30% blessed resil passive, but on the flipside, I rarely get targeted first as a monk, and even if they do target me I have some great escape abilities.
So yeah, I'm recommending MW monk or holy priest. Holy paladin and resto druids are very solid as well, but neither will have the throughput of a MW or h-priest. GL and please don't be bad :P Use mouseover macros :D