If Peyton plays, I think you have to start him. As far as I know, that's the case, right now. Revis will slow the Colts' passing game down a little, but 4 quarters of Peyton is still better than just about anyone out there.
However, if news starts leaking out that the Colts' will pull starters for any part of their Wk. 16 game, then I think you go with Favre. I do expect the Vikings to lean heavily on Peterson, but Favre should still have a respectable game against a team with nothing to play for. And I think he's definitely a better option than a guy who was temporarily pulled for Trent Edwards in Wk. 15, even if the opposing defense is weak.
Best of luck!