Views on leveling to 80.

Holy:
Mana intensive. You have more AoE healing but it will drain your mana very quickly. When I needed to AoE heal a lot my priest would go OOM and I would lose the fight anyway. I had to really manage mana very carefully and drink after all but the most trivial encounters. Maybe my priest was just not geared properly for Holy.

Disc:
What a difference. No mana issues at all. I think my priest went OOM only during Insanity and even then it didn't matter as the priest got enough mana back to cast some spells that generated even more mana. Most fights the priest end up with almost full mana. Holy Nova spam is great and I think my priest with current gear will be able to sustain healing for much much longer than Holy. The entire Disc tree is just reduction of mana cost and extra mana procs, it is insane.

If you have any mana problems at all spec Disc and you will never look back.