If you are in the process of spamming the hell of heroics, I'd highly recommend you go Holy. I've seen so many undergeared Discipline Priests in heroics, especially the new ones, struggling to keep the group topped off through some of the more intense AoE moments. Circle of Healing plus hasted Prayer of Healing through the Serendipity talent makes group healing, especially when you only need to worry about 5 people. I know discipline might feel like a more "fun" healing spec, but coming from an experienced priest - heroics are so much easier as Holy. Just something to keep in mind.