Sell. There are too many players leaving in 2012/2013 that aren't a big part of this team's future, and they'd be worth a ton to other teams.
In order to win the division, the Red Sox will need to go 47-26, with a .643 winning percentage, and the Yankees wil have to go 37-37. So the Yankees will have to collapse, the Red Sox will need to turn into complete juggernauts, and the Orioles, Blue Jays, and Rays will all need to wilt and disappear... except in AL East games.
It just doesn't make sense to risk all those beautiful trading chips for a winner take all 1 on 1 matchup with 2012 AL Cy Young Winner, Jared Weaver.