Nothing beats DK's in PvP and nothing beats Shamans in PvE. There's no solid information that this is going to change in MoP. Ret and Shadow look like they're getting buffs, but nothing I've seen indicates they will displace Frost DK's or Elemental. Any levels you grant would just be a guess at which class may play solidly after Blizzard is done playing musical chairs with game balance.

Since you don't have a lot of time to play, I'd just pick whichever class you like the sound of to dump the free levels into (no point wasting them, since they expire when your 90 day RAF ends) and invest your time into gearing your Shamans.