BF3 after the most likly 10hr campaign is done, you still have multiplayer online with much to unlock and achieve to motivate you to keep playing.
Skyrim, campaign has to be very lengthy since Oblivion cost me 300hrs+(not completed lol) of time. But if you rush the campaign im not sure what the MP is in Skyrim but I would assume it will die out faster than BF3.
When faced with $$$ to decide which game to buy next, I always look at replay value.