If its a choice between Priest and Shaman, for a team that already has Shaman on it...
I would run with a Priest over a Shaman.
Given you already have three Shaman on the team, so most of the totems will already be covered.

A Shaman is a fine healer, with Earth Shield and Riptide.
And if you don't have another Shaman on the team, Heroism/Bloodlust can be a major factor in their favor.

The Priest gets you Fortitude and Fear Ward.
You have a choice between Holy (big heals) or Discipline (mitigating damage), which essentially ends up being about the same.
So its a preference call.
A Priest is a very mobile healer (like a Resto Druid is), and is probably the best AoE healer possible.

I'd also recommend dual-specing the Priest and Shaman, if you go that route.
That way, you can try with Resto/Shadow and Holy or Disc/Elemental, to see what your preference is.




Personally, I'd go with a Warlock, although a Boomkin is an option.

I would also strongly consider another DPS class that provides the equivalent of +13% spell power.
I'm not sure if the amount is still 13%, but that's what it was in WotLK from Boomkin/Warlock or Unholy DK.

My favorite PvE team was 1x Paladin (tank), 3x Elemental, 1x Warlock.
The Warlock provided some fun buffs, was comparable DPS to any of the Shaman on the team, and added the +Spell Power, which was really strong.

Three Elemental, can (or at least could) heal any five-man content as well as an equally geared 1x healer could.
Having the fourth DPS, really sped things up, for anything that was on farm, especially the trash clearing.
But they've given all of the dedicated healers a strong cleanse mechanic, which you only get with the Holy/Resto spec.
You'd be giving that up, if you went 3x Elemental and 1x Boomkin/Warlock.
Or you could dual-spec one of the Shammies to Elemental/Resto, for when you need that feature.
Kind of like... dedicated healer for harder stuff, but 3x DPS (who can heal) for easier stuff.

Warlock: Group summoning, Healthstone, Soulstone, Pet Buff, Easy to Macro, +SP via a curse, Fear, CC vs Elemental/Demon.
Boomkin: Tranquility, Innervate, Thorns, Battle Rez, +SP via regular DPS spells, CC vs Animals, Root, AoE Spell Interrupt talent.
You already have some of the Boomkin buffs, as a Feral tank.