There have been a few very significant changes to the priest healing mechanics in 4.0. Of course, every class got a few new abilities and likely had a few nerfed or taken away completely. However, priest’s gameplay style was changed in a few aspects.
For one, the talent trees are quite easy to choose from and make cookie-cutter-like now. This being said, I believe every player can figure this out and I won’t go into further detail on choosing specs.
Gearing is pretty constant for both specs. Make sure to stack intellect primarily. I will go further into which stats affect which spec and how.
Discipline is mostly the same. Disc priests still play the same way – they bubble everyone they can, and proc out additional bubbles via crits from penance, holy nova, and prayer of healing. Of course you will still want to do all of this. You will still have access to pain suppression and power infusion, which seem unchanged. One great additional added feature in 4.0 is the Power Word: Barrier. This is a 3 minute cooldown ability which gives a group of people a massive amount of mitigation. The animation for this ability looks similar to the Death Knight’s anti magic zone, only, it is priest-like and resembles the animation of holy nova. With a very wide area spread, many people may fit in this zone to get the full effect on multiple players, or one person, typically a tank, can get one large absorb from the bubble. The uses of this bubble are endless, and I believe the amount that the bubble absorbs does in fact scale with spell power.
Mana efficiency is still very good with disc, if not better. Since spirit now affects disc priests positively for mana regen, it is no longer necessary to fight for avoiding this stat completely. You can now use the same gear set for both holy and disc, if you wish.
Holy has received some amazing play style changes. The new Chakra and Chastise abilities are very unique. Of course, most of the play style is the same for holy priests – use circle of healing on cooldown, keep prayer of mending out, and otherwise renew as many players as you can. Chakra adds another feature. Chakra is an ability that is activated, and when another spell is used opens a window of options. If you use Prayer of Healing with Chakra activated, you will go into an AoE healing frenzy, which will decrease the cooldown time of your circle of healing, increase all area of effect heals by 10% health, and turns your chastise ability into an AoE heal effect that goes on the ground for low mana, cast time, and on a short cool down for the entire time you are in the chakra. If you cast renew will in Chakra state, you will enter a state that drastically reduces the global cooldown of renew, increases its healing by 10%, and gives you a second, very powerful heal over time effect on a short cooldown. If you cast heal while in Chakra state, you will enter a phase that allows the heal spell to refresh renew on its target, as well as gives you increased healing to them, and a relatively strong instant cast healing effect on a relatively short cool down. You could see where each of these effects would be useful in a given situation. Also, remember that Chakra does in fact last 30 seconds after its cast time (with the cooldown being 1 minute), but if you use the same ability that was used to activate it while you are phased, you will increase the duration of the ability (meaning if you use the same spell enough, you could have 100% uptime chakra phase on that ability).
The mana efficiency of holy has probably increased since pre 4.0, but disc priests have far better mana efficiency than holy priests. Of course, this is a play style choice and you may prefer holy thus don’t care about the mana problems you are likely to incur. If you’re all about having a lot of mana available though, you may not want to play holy, or you may need to stack efficiency stats over throughput.
Overall, both specs are viable per situation, and it’s more of a playstyle preference than anything. I personally play and love both choices.