Both are great, mages have a better record for being great consistently but honestly it doesn't matter much. All 3 specs of both classes are better at certain ilvls but ultimately you will be fire for mage and aff / destro for lock.
All that said I think these kinds of threads usually get locked.
I was watching Azael stream last weekend on his lock. His team mates ended up bailing on him and picking up a mage to do 3s with instead. If one of the best locks in the world is getting benched, then it's safe to say go mage right now.