3x Shaman will be easier to run.
But going 1x Boomkin or 1x Warlock with them, will get you 13% spell damage, which is very nice.

Since you already have a druid as the healer, I'd probably go with the Warlock.

You already have a mixed team.
Paladin + Druid + Shaman's, so you'll be doing click castsequences anyway.
And a warlock is easy to set up a click sequence for, with either spec.