No such thing as an ICC Cata raid - ICC is a WOTLK raid... and nobody really runs it anymore except for acheivements.
Discipline priests main "power" is their shields... but you cannot have two shields on a target at the same time, hence you only really need one per raid. They are good single target healers but have limited AOE heal's. Personally... I loved Disc, but I found mana a real issue during intensive fights.
Holy on the other hand is more flexible... you still have a shield, but you have a much better AOE arsenal... It takes a lot more getting used to and requires more thought on how to react to each situation. The main ability is the ability to increase your crit on single target heals or output some big AOE heals - but there is a cooldown on using this. The reason why I think it takes more getting used to is that I STILL haven't got used to healing with it (despite it being me 2ndry spec)... i usually end up never using my AOE abilities as I probably should.
Holy is probably more powerful overall IMO, just from a pure felxibility prespective. But for a raid (which I don't do) Disc would probably be preferred as a tank healer. As a Main Tank healer you are also competing with Pally's (which as I understood it, were exceptionally good MT healers), whereas Holy priests have a lot less competition for AOE healing... I think Shammies would be the next best AOE healer, but I know a lot of shammies complain about their abilities compared to Holy Priests.
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Why use Ambush?
Your foe is in cloth and you want to give them a nice surprise!
- Sprawl