I would go to San Francisco. Yes, Alex Smith had a good year, but I have my doubts that he can do it again.
They have a good o-line, and they have Vernon Davis, Frank Gore, I'm sure Manning will make their WR's better.
And their defense was pretty good too. They had a ton of pro-bowlers last year- with a decent QB, they could win the Super Bowl.
I'm not sure how much free reign Harbaugh would give Peyton like the Colts did though.