Mage mage would be straight forward DPS, tho you could use one as a scorch stacker and the other fireballs. Or one uses frost and the other fire so it takes ages for a mob to get there.
Lock priest would be dots and fearing, priest adding to locks DPS with the added bonus of being able to heal.

Personal preference. I'd go mage mage just for the DPS, ease of use, and cause you then get 2 sheeps, lots of aoe and much pain.