First of all.. the weather is overrated. Miami has more rain than Seattle.
http://www.komonews.com/weather/faq/4348261.html
Second of all.. The Seahawks play in a much easier division. They are similar to the Colts in that the media doesn't really care about them as much as they would Miami, Washington, or the Jets and Peyton craves that.
Third of all.. we have the best facilities in all of football, a owner who will open his pockets and get Manning help where he wants it, and a player friendly coach in Pete Carroll who knows how to recruit.
Lastly.. lol at saying it's just Marshawn Lynch. They have Doug Baldwin, Sidney Rice (when healthy you saw what he did with Favre), Zach Miller, likely Reggie Wayne if we signed Peyton... and a good franchise LT in Russell Okung and an explosive young defense that will get better in the draft and free agency.
Seattle is a very attractive option.