Disc priests can be fairly mobile.. No where near that of a druid, but good enough. I can't comment on being holy as I never played a holy priest but with my disc priest so far I love her. I haven't put her into a situation where enough AOE damage has come in to risk my team. For expected AOE that comes in timed bursts, holy nova then a renew is usually enough to top off my team.
Not sure how that would hold up to constant AOE damage. Holy priests definately do better in those situations.