I agree QBs don't play in a vacuum. But Colts fans always use the "Peyton didn't have the defense Brady had", but there is just as strong of an argument (probably a better argument) that Brady hasn't put the numbers up like Manning has because he had substandard weapons on offense. Seriously name one receiver who left the Pats and done anything. Branch even when he was healthy was sporatically good at best. The rest has been less than stellar. Brady has only had receivers of a quality similiar that Manning has had his entire year in 2007.
This point is valid up to a point if you want to apply the same standards to both QBs. I mean, how did Manning's receivers do after they left the Colts? You can't say because they never left. Great QBs in great offenses on great teams make everyone look good. You can speculate how good Marvin Harrison would have been on a different team, but you can't prove it. You can't prove how much better Manning made Harrison look, or whether it's the other way around and Harrison made Manning look better. It's pure speculation.