I think you *could* pick Sheffield and have maybe a 70% chance of him going .309 37 115 105 4 (as the above poster mentions), and maybe a 30% chance of having an injury or rapid decline.
On the other hand, you could pick Chase Utley (assuming he's still available), a player on the upswing of his career, and have a very good chance of him going .295 30 100 15, at a much scarcer position.
Me, I'd go with Utley. If Sheffield is slightly better stat-wise, you'll console yourself that you don't have Ryan Freel or Todd Walker as your 2B. If you pick Sheffield and he falls apart, you'll be much more upset.
On the other hand, if Utley gets injured and Sheffield hits 50 HR, you'll be kicking yourself too...