I voted for four mages and a priest, simply becasue of the massive ammounts of spec-comboes. You've got:

4xPompyro (meaning arcane+fire) mages + one holy/disc priest for massive AoE and burst damage.

4xFrost/elemental/frost+arcane mages + one holy/disc priest for massive CC and some epix damage

4xFire+arcane mages + one holy/disc priest for loads of burst damage and some half-decent CC.

Of course, you could always spec the priest shadow for a wee bit more damage, but the healing aspect really outdo the tiny DPS you'll be bothered to do anyways. Besides, post 2.3, holy mages have insane manaregen, thus making them even more efficient. I have one in my 2v2 team (warrior/priest) and the guy just never ever seem to OOM unless he chain-manaburns.