Druid gets you 13% spell power, just by casting Wrath or Starfire.
You can get the same from an Unholy Death Knight, or any spec Warlock via Curse of the Elements.

The druid will require managing Eclipse procs, for maximum dps, which can be a pain.
I'd go with a warlock on a heavy caster team.

Personally Prot Paladin > All, as far as the tank.
But that is a preference thing too.