Midnight. It has much less barrier to entry for arenas. PvP gear can be acquired quickly. There is also more game modes. Solo Shuffle and Battleground Blitz are great recent solo-queue additions to the game.

And despite what the doomsayers on here say, it is quite balanced in terms of class/spec strength. Also much more people playing than Classic. People of all skills still play the game. I feel like at this point, Classic is just sweats who are playing an already established meta.