Yeah, it's almost impossible to separate the two, and you can't have it both ways.
I claim that it's because of Belichick. The evidence (and really the only possible evidence to break the tie) is that the Patriots still went 11-5 with Matt Cassel.
It's bad reasoning.
1. They went 18-1 the year before. So there was a 7 game dropoff.
2. The year before, they faced 10 playoff teams during the regular season. In Cassel's year, they faced 2.
3. In 2010, Cassel had a much better year stat-wise with the Chiefs than he did with the Patriots in 2008, one of the league leaders, and he even made the Pro Bowl.
People keep talking about the Indy dropoff from 2010 to 2011 but what about the Indy resurgence to 11 wins in 2012? Surely, you can't give Luck all the credit. Isn't it likelier that they were tanking on purpose with Curtis Painter in there? I mean, they put Orlovsky in there at the end of year and he actually won two games (after the first pick was almost assured).
Here are the killer facts for me in comparing the two:
1. Brady has a higher passer rating outdoors
2. Brady has the higher passer rating in domes
3. Manning has the higher overall passer rating
When you put all these 3 facts together, you really start to wonder a little bit about the advantages of playing in a dome.