So I'm thinking about coming back to wow, I played played a disc priest from wrath until MoP, how how are they doing this expansion? is it worth boosting my priest or should I reroll something else?
Completely different, disc now heals by DPSing. I'll let someone else give the details about balance, but the consensus is that they are overall the hardest healers to master with a few really rewarding AoE burst moments when you do master them
With an astounding array of versatility, Discipline Priests are an exceptional healer to bring to a raid composition. Providing a huge damage reduction cooldown that is jaw droppingly effective at countering many mechanics the legion can throw at you that can only be paralleled to a stronger version of Spirit Link. However, that is not the only cooldown Discipline offers, they also have a huge revival type cooldown in which a well timed blanked of atonements in the raid can lead to you soaring through the meters. Discpline has seen many uses in high end mythic progression, however the only downside to raid healing for the specialisation lies when there is no target to attack as it heals through dealing damage and applying buffs to the raid. Furthermore, the spec is only competitively relevant on the meters if your raid has one or two boomkins to supply innervates (additionally, a blessing of wisdom from a ret pally does wonders!) as disc essentially converts mana - into HPS, causing to suffer without mana saving sources from other allies! All round, disc is the most fun (biased), rewarding and challenging healer to play that will take some runs to master. Knowing the fight mechanics is essential to a good Disc Priest as it is a pro-active healer rather than a reactive healer like its counterpart Holy which will ensue in the next chapter........
Holy..... So, you want to play holy. I do not blame you, Holy has served as one of the OG healers of wow and is certainly one not to be trifled with when it comes to topping the healing meters. However, fights with considerable movement shun the Holy Priest down as it is very much a stand still and chain cast heals spec with some instant casts in the mix. Holy is a specialisation that you will be comfortable with from a pick up and go spec, whilst offering a more rewarding gameplay for experienced players due to the mechanic of your holy word 'mini' cooldowns, as casting normal healing spells reduce the cooldown of your mini cd's allowing the Holy Priest to pump bursts of healing when necessary. Holy does not suffer from the mana complexity that Discipline does, however it offers a more raid effective healing cd like tranquility with Divine Hymn, but on more challenging content, a barrier from disc is what saves people from being one shot, so it is all about composition, and boy let me tell you, every guild is always on the look out for a good Disc!
TLDR:All in all, the two specs are very good and both very fun. Good raid utility and viability.
Faezer already made a very nice summary of each, i would add this:
Holy:
+ huge HPS, very good on fights with constat AoE dmg
+ huge targeted burst of healing with mastery acting as a top-up (both AoE and ST)
+ versatile, you can switch styles on the spot. Talents help you to specialize
+ good raid CD, good ST
+ can sustain healing with 0 mana
- immobile
- low personal def CDs
- dumb heals, without paying attention to the position of your raid members you will end up overhealing a lot
Disc:
+ amazing spread burst AoE healer (stacked as well)
+ multiple raid CDs, can be extended with certain trinkets
+ slightly better ST CD than holy and more personal survivability
+ preventing dmg through Shields (cheesing some mechanics like Varimathras), without becoming a shield bot
+ dmg reduction through Atonement effect and also mass dmg reduction CD
+ mobile
+ good dmg while healing
- preparation is required for healing, usually 5-10 sec before the dmg happens
- scales negatively with raid size (good for 20 man, but the higher you go the harder is to maintain Att)
- single target is limited and usually requires Grace (talent) to be even close
- mana management required (+ but they can tansform mana into healing very very well)
For Legion they are not Legenary dependent but having Velen helps on both. Having good loggos will improve your play style and numbers
PS: Spirit link is mechanicaly superior to Barrier, due to the HP balancing effect on top of the dmg reduction (lower, but present). It also helps the other healers in being more efficient with less OH