According to a good number of sources, namely Elitist Jerks, Disc Priests are absolutely wonderful for fights such as sarth+3. But, most of the Priest community will probably react to the switch as I did at the beginning of Lich King- Disc? No way. PvE healing = Holy. However, this is no longer true. Even if you're in a guild that is still progressing through Naxx, or even if you're a PuGger, Disc Priests can be wonderful things to carry around in your back pocket.

Whats the difference between a Disc Priest and a Holy Priest?
Holy folks are wonderful at repair. We are the Jack of all Trades [Master of None /cry] healers who have a variety of single target and group wide heals. We have direct heals, HoTs, shields, insta-casts, long casts, everything. And we use everything. If you're healing as Holy and you don't have every single healing spell [with the exception of the leveling ones such as Lesser Heal and Heal] on your action bars then you're wrong. I don't care what your excuse is- Holy Priests use every. single. spell. Yes, many of us have become Flash Heal spammers but we still use...everything. You start the pull with a PoM and refresh it everytime the cd's up, shield a threat-happy DPSer, CoH or PoH after aoe damage, Flash Heal or Greater Heal depending on the damage taken, Binding Heal when you're hurt also, toss Renews...Guardian Spirit in O-S! situations...etc. Everything.

Discipline Priests are all about mitigation. They have a variety of shields and damage reducing spells that prevent oncoming damage. They will probably not top the healing charts...ever. Because they don't really...heal. They make the other healers in the group lives' easier by saving them mana and GCD's-making the others all the more efficient. Of course, they still have all of the wonderful spells a Holy Priest has, they're just not as good. A properly specc'd Disc Priest will go alllll the way into Disc, picking only a few talents out of the first two or three tiers in Holy. They don't get the extra healing, or spellpower from spirit, or mana cost reduction or any of the other strong Holy talents, but it's ok- they don't really need them. No, this does not mean a Disc Priest sits in the back of the raid tossing shields and never casting a heal...sitting there idly bored- maybe reading their favorite web comic. They have to know the fights just like any other healer because spells have several minute cd's, and a wasted one could cause a wipe.


So, which is better? Mitigation, or repair?
Both.
o.O
Mitigation makes it easier to repair. Multiple Discipline Priests are not really necessary for a raid, but having one around makes all of your other healers happy. Deciding which you want to play is all about...how you like to play. Holy Priests are generally in the middle of it all, tossing out heals as fast as they can be cast, while Disc's stay to the back and watch for possible incoming damage. At present, I'm still Holy but that's only because another Priest geared for Disc faster than I did ><

Speaking of Gear...
Holy Priest's aim for the max healing output with the highest amount of mana efficiency. Mmmm...spellpower, Spirit, Haste, and mp5! Crit is a good side item...int is important, but mana regeneration is strong as Holy so it's not really something to focus on.
Disc Priests are kinda like Pallies. They don't really have mana concerns, they love crit, and they don't benefit from Spirit in any way other than mana regen. Spellpower, Crit, and Int are the most important stats for Disc folks.

Using Loot Rank, I've compiled the following list of gear choices for a Disc Priest. I only chose to display the top 3, but you can copy the values and choose a longer list. If you prefer one stat over another, it's pretty simple to change stuff around.

Disc Gear List

As you can see, alot of the gear is more DPSy than Healy. But, that's because most cloth gear with crit is aimed towards DPSers >< I promise I won't make fun of you if you're wearing the Shadow tier gear.

If you have any questions, feel free to email me or post 'em here!
♥Fae