I would keep leveling affliction on the lock until 50. takes a few seconds to kill something but its great survivability. If it was me, I'd switch the lock to be the main @ 50 and respec demonology to get the felguard and run the priest as holy or disc and aoe farm / quest. Just round up mobs with your pet and keep your priest healing the pet. If you pull aggro you're priest is right there for the instant fearbomb.

I say run the Lock as you main because next to the druid class, I've played a lock the most, so I'm pretty comfortable where as I'm a noob with a priest other than being a healbot.