Originally Posted by
VigilantRose
It's complicated.
Priests have a much better basic mana structure. They get Spirit/4 regen vs. Spirit/5 regen for Shaman. They get a talent that lets them retain 15% of that mana regen while casting (and later, gear which ups this to 30%). They also get a talent that converts Spirit into Spellpower, creating a situation where Spirit is actually a good stat for them while Shaman tend to avoid it. So if you look at a Shaman and a Priest, the Priest will normally choose far more Spirit - and thus be far better at recovering mana when not casting (as well as while casting).
However, that's not the whole story.
Shaman are much better at downranking because they can do so more effectively due to the fact that they have a single main heal while Priests have three different versions. This means that the foundational Shaman heal is (Rank 3) of Healing Wave. It's their most efficient heal and - because it's the same as their top rank heal - it has a 2.0 sec casting time. In contrast, Priests would have to cast Lesser Heal to downrank that far and they'd suffer an enormous throughput loss that Shaman don't face because they don't get a casting time reduction on Lesser Heal.
The counterpoint to that is that a Shaman's Healing Wave isn't all that effective without the buff Healing Way, which has to be placed by Healing Wave itself. Coupled with Ancestral Healing (Shaman version of Inspiration), this tends to mean that Shaman spam-casting like Paladins. To make their Healing Wave viable, they need to stack Healing Way and to stack Healing Way they need to cast a lot of Healing Waves.
Now, let's step into a Horde-side raid. You have two basic healing demands: tanks and everyone else. For our purposes, we'll ignore the fact that Druids exist since you're not bringing many and they aren't particularly good at healing anyway.
So who should heal the tanks? In terms of who is better at it, the answer is easy: Shaman. Once they've got that Healing Way stacked up, Healing Wave is a better spell than Greater Heal. Not by a huge amount, but it's still better. So let's ask the other question: who should heal everyone else? Here, the answer is even easier: Shaman.
Unless you're talking a situation where you're dealing with mass raid AE and Priests spread across the groups, Chain Heal is significantly better than any option Priests bring to the table. Since Priest=tank;Shaman=other is better than Priest=other;Shaman=tank, you end up with Shaman pounding away with Chain Heal.
Unfortunately, while Chain Heal is efficient, it's not enough more efficient than single target heals to counterbalance the fact that - most of the time - 'everyone else' takes enormously more damage in aggregate than 'just the tank'. So if you've got all your Priests on the tank and all your Shaman Chain Healing, your Shaman will run out of mana a lot sooner.
As an intermezzo, we'll also note at this point that Priests get much better gear options than Shaman. For example, Priests get the only non-PvP epic +healing weapon until Phase 4 (Benediction). The Dire Maul trinket for Shaman is a gimped Briarwood Reed; the trinket for Priests is a BiS choice until AQ40. And so forth.
Now, we need to talk about Tier sets. T1 for Priests is, well, let's just say it's pretty. T2 for Priests is fantastic. However, tier sets for Priests are itemized purely for healing.
Shaman tier sets, on the other hand, are itemized either for a hybrid of healing/spell offense or almost purely spell offense. But while Priests have some decent set bonuses, Shaman get bewilderingly overpowered set bonuses.
With T1, wearing all 8 pieces means that a deep Resto Shaman reduces the cost of Healing Wave by an incredible 25% on average. Plus they increase the throughput of their already best-of-class Healing Wave by 24%. Plus, they now can spread Healing Way like crazy across a raid. If you look at logs, you can immediately identify the 8p T1 Shaman by the fact that they cast nothing but Healing Wave. The 8p is so strong that Resto Shaman will still be in full T1 in Naxx. And they won't have mana problems because they're able to spam Healing Wave (Rank 3) forever like a Paladin spams Flash of Light.
But there's a problem here. Remember when I broke down "who heals what?". Those 8p T1 Resto Shaman will be generating a lot of raid healing - it basically makes Healing Wave into Chain Heal. However, it's very slow, unbalanced and short range. That means it's not very good at, say, keeping your Mages alive. They just don't have enough hit points. If you have all your Shaman go with this sort of 'deep Resto' approach, it leaves a huge gap in your healing structure because Priests are abysmal at the kind of spot healing you need to keep your non-tanks alive.
So you can maybe afford 1-2 Resto Shaman gearing like this.
The rest of your Resto Shaman are gearing in a "3/5" configuration - the bracers, gloves and belt from BWL coupled with the entire set from AQ40. This makes their Chain Heal much more effective. However, because Chain Heal only has three ranks, it can never reach either the efficiency of downranking nor the throughput of upranking that your deep Resto 8p T1 can manage. Moreover, the '5' part of that '3/5' is basically Elemental Shaman gear, so you take a big hit in +healing to equip it.
Now, you step into a 20-minute Naxx raid. Your deep Resto 8p T1 Shaman can heal forever - but the peculiarities of their healing means they're not nearly as flexible as a Holy Paladin would be in that role. Your "3/5" Shaman (who, frankly, are probably Elemental at this point because the top end of the Resto tree does little for them and they want to PvP) can raid heal very effectively, but they can't do it for that long because they can't downrank below level 40. Couple that with the fact that they don't get the kind of mana infusion Priests do from standing around doing nothing... and they've got to be careful with their mana.
That being said, most healing right now is just spamming small, quick heals like Flash Heal and Lesser Healing Wave because fights are so short in MC with geared dps. Shaman and Priests tend to be equally effective at this sort of healing - Priests never get to exploit the FSR for superior mana regen and the two heals are roughly equal in quality.