I think it should be Wiggins, purely based on the fact that Murray will have more years and opportunities like this, and Wiggins will not.
In fact I reckon Murray, now he's won his first, will win at least one more Slam next year. That should be the SPOTY in the bag for him in 2013...so let Wiggins have it this year. Don't really think anyone else is in the frame to win it.