If discipline and holy "healing output" would be equal - as blizz says they are aiming for, then: Discipline will always, in every situation be better then holy.
I quit the game after 5 years of playing a holy priest, the reason i quit was cause i got fed up with how casual the game became: how spamable, mindless, and casually easy everything has come. I remember a game where geting epics was actually hard, and seeing a player with epics was actually rare. Even in TBC completing heroic runs (like shattered halls) used to be a hard assignment - and you had a group of 5 who had to think before pulls, plan how to kill bosses. Eveything atm is just tank and spank, and spam heals. It doesn't matter what you spam.
Holy fell alot behind all the healers in WoTLK, especially behind the discipline priest who does have a spamable rejuv/HL style. Now healing outputs will be equal:
meaning: discipline will heal like holy, but will also absord damage and have the spamable shield and barrier.
WoW started as a amazing role game, where you picked a class because of what the class gave. Now all the classes are the same, the difference: name of spell and armor.
With the new shaman changes: all i see is the same WoTLK line: shamans are like priests but they have totems, BL and mail armor. Paladins will probably also be priests with blessings, aura's, hands, bubble and infinite mana, Druids should've been nerfed at the begining of WoTLK (i still can't believe how a druid can cast ONLY rejuv and still be uberly good and amazing), Discipline priests just spam shields and are considered SO good because of it. And holy priests: there are the ones who compete meters (which are common on mmo hi kelesti nub) and some that actually play properly. At the same time - a holy can't really compete atm on meters - set bonus, no spamable strong spell, its more of combining all together to get this.
Very dissapointed not planing on coming back, waste since i got the invitation to test.