This is the way we set up our healing in 25 man.

Priests are shadow. They rock have 3, its just insane. You might want one healy priest in a raid though, choice i guess.

3 Paladins, they raid heal normally.

3 Shamans heal MT mostly with chain heal thereby healing the melee group, which usually takes the most damage.

1 Resto Druid HoT's the tank(s)/ raid heal.

With this setup resto shamans always top the meters. Shadow priests with resto shamans = low threat or more damage/healing and mana and no one runs out of mana like ever. Resto shamans help everyone. Paladins are very good at raid healing with short cast heals with to keep up people getting random damage. Priests help everyone else in the raid keep up with VE, and Shamans chain heals keeps up the melee.

Without getting into the who is better debate (because you need all four in a 25 man anyway) I'd like to point out the druid's bonuses since there are a few that have been missed.

ToL buff makes everyone easier to heal by 25% of the ToL's considerable spirit.  Lifebloom rocks for aggro management (boss fights this isn't a problem, those %^&$ murlocs though.... and all other trash mobs...) as it tosses threat on the one healed instead of the healer and is still insanely mana efficient (though one should be careful about LBing the dpsers for the same reason - they don't need the added threat any more than you do).  Nature's swiftness makes Regrowth instant cast and Swiftmend makes Rejuvenate more mana-efficient and faster than Healing Touch -- not that you will be using HT anyway since you can only use HoT's in ToL form.

Being able to remove poisons and curses is invaluable in some fights and GofW makes everyone better at whatever they do. Buffing and removing debuffs are primary jobs of healers.  (Obviously other classes' buffs/debuff removal abiities are equally important since there are still diseases and magic debuffs to contend with that druids can't touch.)

Getting the fallen back into action while the battle is still raging is essential when the battle is fifteen minutes long (or more) -- especially when people need to be alive when the boss falls to benefit.  This is perhaps one of the reasons why druids are so very sought after (and the ToL/GotW buffs).  Other classes can heal as well or better, but Rebirth saves groups again and again.

Well-geared druid raid healers rarely OOM - though as mentioned, Healadins have good mana efficiency too.  ToLers have insanely high mana regen thanks to their high spirit and learned talent -- and in a pinch there is always innervate. 

Now, the other three healing classes have their own equal place, so don't think for a second that I'm saying druids are superior.  Just wanted to lay their finer points out there.

On the downside I spend a lot of time staring at HoT/rotation timers and pushing the same three keys over and over with the occasional fourth (Nature's Swiftness or Swiftmend) added in for uber variety. (/sarcasm)  Not sure that other healing classes have a more exciting job, though the Shaman looks very cool and priests get to wand during their down time.  It doesn't really add that much dps to the group, but it beats the boredom of standing there like a... tree.  I've had entire runs where I didn't even show up on the damage meter at all, which is somewhat depressing.

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All that being said, my advice for building a healer is to find the class that you most enjoy playing.  After all, you have 70 lvls ahead of you (and a bit after 70 to get geared up and keyed) before you can even think about raiding, and you will still likely play your raid healer outside of a raid group (PvP/dailies/farming for rep/mats/gold) even afterwards so it needs to be FUN.  Healers are always in high demand, so no matter which class you pick, if you bring skill, enthusiasm, teamspirit, proper spec, and decent gear to the table, I can't imagine that you would have a problem finding a raiding guild that will be willing to take you.

Best of luck!