Probably too close to call. A holy or disc spec would be faster for both though. Rogue and warlock can regenerate their dps (energy, manatap from heals) while fighting while a shadow priest will have to consistently sit if grouped and not getting the killshots.

Lock would be best as affliction and with a healbot priest, not shadow. Shadow will do well at 70, but not before unless you feel like drinking. I have dualboxed warlock priest and have tried several spec's when I grew curious which would level faster. Affliction/holydisc by a mile.