Peyton played statistically the easiest schedule in the NFL this year (tied with Chiefs). They have only played two defenses the entire season that are even above average (in the top half of 32 defenses)- Baltimore in Week 1, and Kansas City which was completely exposed by the end of the season. So that argument goes both ways.
The Seahawks have played a number of elite defenses, including most of the second half of the season. That said, the best offense they faced all year is the Saints. So again, the argument could go both ways.