First of all i want to say that i dont use priest on multiboxing team but as my main char (and first) has been priest already like for years i know about their spells and abilitis. I have not raided with her on 3.3 but before this patch i was active raiding holy priest. With 3.3 i started boxing and left raiding alone as it messed my IRL to much.

As for now 3.3 you should hold Renew up on tank all the time. It dont even madder if your holy or Disc. I myself prefer Holy as it is my play stile but time-to-time i switch to Disc. As for priority its the first spell you ned to hold up is PoM (mending), it is the best almost aoe healing spell priest has. You can use it on the move and it will jump to the lowest HP target (must be in range) so it will prevent most likely hes death. Second spell is Renew. Then its flash or greater heal. Why renew? because it can already heal your target when you cast your next flash or greater heal, and with emblem gear you should have like 1,5k ticks incoming.

If you are speced holy i would keep CoH as "Oh Shit" heal, as it takes a lot of mana but is insta cast AoE heal and for priest itself you will have his disc tree insta cast self heal. Shield is always there for you and you can cast it multiple times. And dont wait PoM to use hes all charges, if its up cast it to the most needed player.

If some players dont think gearing holy or disc is not different then i am not one of them. The items may be the same but as for stats its like spirit vs. int. If you like Disc hold for int, if your holy hold spirit. But never overdo it. I am trying to keep its like 1200 int /800 spirit for the moment. As disc i would go more for int and hold that spirit in lower levels. at that level of spirit you should get like 200+ Sp as holy from spirit, but you loos it as disc and it has way less use from it. Disc manaregen uses more int as it is binded with how much you have overall mana. This stat comes from int. So if disc you should keep like around 1200int / 600 spirit. I would not use MP5 gear unless you dont have better on that slot. Haste and crit are nice stats to have on priest but i would get them from gear only and not use stones. If you have shammy or boomkin in party you will already win 5% crit and haste from totem, so it should be ok.