This is an intriguing hypothetical. Snow would seemingly level the playingfield... Or tip the balance to New England.
I wonder if someone could find comparative statistics between Manning and Brady in wintry conditions? (I would research myself but do not have the time and am posting off my smartphone/3G) I would expect Brady's to be much better and the sample size to be larger.
I don't have the time to go through every box score and see what the weather conditions were. But let's just create a general "bad weather" scenario: Any date after Nov 20, in a cold weather, outdoor site. That should work for our purposes.
Here are their career #'s (including playoffs) in those types of situations:
Tom Brady
71 g, 64-7 (.901), 28.3 ppg, 1465-2318 (63.2%), 17227 yds, 129 td, 42 int, 96.7 rating
Peyton Manning
26 g, 15-11 (.577), 22.5 ppg, 540-860 (62.8%), 6351 yds, 38 td, 28 int, 86.3 rating
Now, let's compare those stats with each QB's overall career totals (including cold-site games) - stats are per-game averages:
Tom Brady
Category - Overall - Cold Site
Win % - .771 - .901
Comp % - 63.6% - 63.2%
YPG - 254.3 - 242.6
TD/G - 1.89 - 1.82
INT/G - 0.73 - 0.59
Rating - 95.6 - 96.7
Peyton Manning
Category - Overall - Cold Site
Win % - .670 - .577
Comp % - 65.0% - 62.8%
YPG - 267.0 - 244.3
TD/G - 1.91 - 1.46
INT/G - 0.94 - 1.08
Rating - 95.1 - 86.3
So clearly, Peyton Manning's numbers are significantly worse in cold weather games than they are overall. And obviously, if you took out their non-cold weather games, and do the same comparison, here's what you get:
Tom Brady
Category - Non/Cold - Cold Site
Win % - .772 - .901
Comp % - 63.9% - 63.2%
YPG - 260.8 - 242.6
TD/G - 1.93 - 1.82
INT/G - 0.80 - 0.59
Rating - 95.0 - 96.7
Peyton Manning
Category - Non/Cold - Cold Site
Win % - .682 - .577
Comp % - 65.3% - 62.8%
YPG - 269.7 - 244.3
TD/G - 1.97 - 1.46
INT/G - 0.92 - 1.08
Rating - 96.1 - 86.3
So they are just about the same QB in non-cold situations, but wow, tehre's a major difference when it comes to cold-situations. Brady is far superior.
So long story shorter, if they play in any sort of cold or wintry conditions in Denver, Brady has a significant advantage there. It's not to say that Peyton couldn't play well, because clearly he could (he's had many good cold-site games), but it's an advantage for New England. Their entire careers bear this out.