Last night, we had a very difficult time with Shade of Aran. In the transition, we lost a lot of healers. And for various reasons (Internet/computer went down for days, RL issues, etc), we have been extremely limited on our healing this week. For healing, we had an undergeared shaman, a moderately geared shaman, and a moderetly geared paladin. We oddly enough had some huge issues with the Flame Wreath. Why anyone was moving when everyone knows that fight and has done it successfully on multiple occasions is beyond me.
So, I thought I'd take a moment today to talk about the different types of healers and how they function, or at least how I understand they heal. I'll be making some over simplifications, but feel free to correct me if I'm flat out wrong. I won't go into the buffs that each brings to a group; that's beyond the scope of this post, but is a good discussion for the future.
This class is the king (or queen) of the Heal-Over-Time spells (HoTs). With the proper spell rotation, they can keep HoTs up on 3 people at a time. Their spells are generally quick casting, if not instant-cast, but they do not have strong direct heal spells. Because of the quick casting times, they can generally be mobile, but have the disadvantage that in Treant form, they move slower. Having a resto druid can help keep a tank up during periods of mass silence. They also have Innervate that can replenish mana for themselves or any target they choose. Also, they have a Tranquility which is an AoE heal but is on a 10-minute cooldown timer. As a warlock does damage, a druid does heals.
King of the direct heals. For the most part, paladin spells take time to cast. They do have the intant-cast Lay on Hands which replenishes health up to the maximum health of the caster, but it's on an hour cooldown and leaves the paladin with exactly zero remaining mana. No HoTs, and no AoE heals. I'd probably equate the rogues damage with a paladin's heals.
From my best understanding, Shamans are one of those classes that basically make everyone else look really good. They have some decent spells of their own, mind you, as well. They've got the preventative maintenance of Earth Shield. They have the extremely slow HoT of a Healing Stream Totem. And they have the unpredictable heal of Chain Heal. But their totems are appreciated by all. (Oops... I said I wouldn't go into that discussion.) From what I've seen and read, it's kind of silly to use the Healing Stream Totem when you can use the Mana Stream Totem (which can effectively translate into more health by way of heals). They probably equate most to, well... Shaman in damage.
Priests have a little bit of everything. They've got HoTs, but not as strong as the Druid. They have direct heals, but not as well as the Paladin. They've got preventative maintenance by way of Prayer of Mending. They can heal themselves as well as another because of Binding Heal. Probably the best AoE heal with Prayer of Healing. They also have Circle of Healing and Lightwell, which both seem like great spells, but are rarely used that I've seen. There's also Holy Nova (another AoE Heal). So, looking at all that AoE, I'd probably have to say their damage equivelant is a mage.
So, is one way better to bring than another? Of course not. They compliment each other and help each other out. If you're in a 25-man run, bring all four. If you're in Karazhan, you're probably going to have to limit it to just 3. Ideally though, it's advantageous to have 3 different classes for your healers so that you have those different abilities. (Plus, they all use different armor types, so no gear is wasted.)