Disc is king for some obvious reasons :
1) Absorbs via Power Word : Shield count as heal in your scoreboard => that permit you to top healing meters.
2) Power Word : Shield is an instant shield with 6 sec CD and absorb almost 1 k damage => it allow you to reposition yourself good if you are too much in melee (after a fear push for exemple) without really casting, also it can tank DoTs.
3) Priest cast time for either damage or healing spells are really short (1,50 sec smite assuming you have no haste...)
4) Penance can be used both offensively and defensively if you take the talent (I'd suggest you this one if you are playing alone)
5) Disc damage is the best compare to others healers even if Monks / Hpal / Rsham / Rdruid do great damage atm, Disc damage is even better than some DPS specs.
6) Penance can be used while moving if you take the talent, that mean you can keep someone alive while pushing back because you lose mid or whatever reason make you push back, or either keeping damage on someone while pushing for fear on opponents healers.
7) Penance while moving can be used, in order to keep healing a target who might LoS you in order to land a kill or when you have to LoS some Polys or Fear's from opponents Mages / Warlocks, or even make the opponent paly run even more in order to HOJ you, or the hunter to perfectly trap you.
8) Penance while moving can be used in order to kill a target who will LoS your cast, since it's a Channeling, it will continue to deal some damage even if the target run behind a pillar, leading sometimes by landing a kill.
9) Schism Talent is really good (in terms of damage) especially if you have alot of healers around you in the BG. It almost permit you to kill shit tons of targets and if targets aren't dead, it increase your damage on them.
10) When you use Power Word : Shield on a target, you know he is gonna absorb almost 1 k, so you can heal somebody else who dropped low aswell, that's why priest is really good at holding multi pressurised targets.
11) Power Word : Shield help you alot with fake casting, because you are most likely safe to not fake cast if you have a 1 k shield on you, you can clearly tank alot of damage while you are kick if your shield is active, i still recommand to fake cast tho.
12) The Shadow Word : Pain can keep rogues in combat everytime and you can also be like a moonkin and spread dot everywhere for doing mad damage charts.
Rsham is really good aswell for some reasons :
1) You run fast : Ghost Wolf, make you good in Arathi / Eye of the Storm, even Warsong Glutch in some extend, also you are walking like everybody if you are getting slowed while using Ghost Wolf
2) Ghost Wolf, permit you to reposition yourself after a push for a crucial shear or Warstomp. (Or either push for it.)
3) Riptide combined with Unleash Life can heal for a decent amount, they are both instant which can help you to keep healing a target while moving back slowly to a decent spot for spamming more casting heals.
4) Unleash Life increase your next heal by a significant amount, even riptide, that mean you can keep you up a bit more without fake casting.
5) The other talent that increase your 3rd healing surge is really good for intense mid fight when you have to standstill and cast. That make Rsham really good in mid fights.
6) Earthbind Totem : Absolutely best AOE slow at the moment, even if they break the totem, a 35 meter range AOE slow lasting 6 seconds atleast. It permit you to either chase or make your team fall back if you are losing a point or the control of mid. It remind me the throwing barrel from BM monk.
7) Wind Shear, with that spell you can negate significant amount of damage or CC's, you can kick Haunt that deal sick amount of damage or either the 4th stack of Arcane even some Chaos Bolt or Moonkins spells. You can also kick heals from opponents team, be careful the 30 meter range can put you in some tough situations, since you have to push in order to reach the backline healers, you will be most likely targeted by opponent team.
8) Flame Shock duration is super long, that make it annoying for rogues. The instant damage is a plus, but you can't spread it like Shadow Word : Pain since it has 6 seconds Cooldowns but it's still really good.
9) Lava Burst deal a significant amount of damage if you are trying to assist your DPS in order to land kills.
That's my opinion, i've played both class since Late TBC as twink, they always has been really good (except cata shaman with earth shield, it was terrible, maybe an exception for FCing...)
