They're very similar as far as what they add to the group.
I'd give a slight edge to the Boomkin on what they bring to the party.
But a slight edge to the Warlock on less things to manually manage in play.

Soulstone vs Battle Rez
-- SS is precast, B.Rez can be on whoever needs it.

13% spell power boost
-- As a warlock curse or automatic from Boomkin dps.

Extra Healing
-- Warlocks can self heal while doing dps or Boomkin has heals spells.

Strong DPS
-- Warlock just does Click sequence while Boomkin needs to manage Eclipse procs.