You got 3x decurse but only your paladin can dispell with a Shaman healer so a priest would bring better synergy but both are good options from my point of view. Hex wont bring much to your group since you got 3x sheep but Divine hymn certainly will! can be a life saver with a bad pull but you will miss a ank thought since your wipe protection is the paladin but he might die before you manage to react.

http://thottbot.com/wotlk/s47951

The priest also brings shackle wich is nice vs undead targets.