While I agree with the general notion regarding Brady/Manning, I do want to harp on a pet peeve of mine:
The Patriots were coming off an undefeated regular season and were going from a difficult schedule to an easy one. They were also replacing Brady with a quarterback who was much better than Curtis Painter.
The Colts, while still a solid and dangerous team, were a team in decline, having fallen from 14 wins to 10 wins over the previous two seasons even with Manning at the helm. The 10 win season had been the worst for the Colts since 2002. So, a team that was in (at least a temporary) decline was replacing an elite QB with a guy who probably doesn't belong in the NFL.
I just see the two situations as too disparate to compare fairly.