I personally feel the druid adds slightly more then the warlock.
They both get the team 13% spell power boost, which is huge.

The druid will have to manage Eclipse procs, to be similar dps to the shammies.
The warlock, any spec, can just cast.. as long as they open with Curse of the Elements.
Much easier to run your click sequence on the warlock, then worry about Eclipse.

Either is a strong team.
3x Elemental, is plenty of healing, for all 5-man content.
Won't need any to spec Resto.
Druid or Warlock will be DPS the whole time (don't drop the Moonkin form to heal).