Other than the weather and indoor / outdoors factor, you also have to consider who they're throwing to.
Brady has targeted the following five players
most in his playoff career: Branch (93), Welker (90), Brown (72), Faulk (65), Givens (64) - only Hernandez (55) is close to that
Manning's
top five: Wayne (133), Harrison (123), Clark (94), Addai (55), Demaryius Thomas (45) - just missing are Edge James (44), Garcon (39), Stokley (39), Marcus Pollard (34)
This is pretty indicative of their careers: 5 of Manning's top 6 were first-round picks while none of Brady's were (Brady's top first-rounders: Watson is 7th with 40 targets, Moss 12th with 25) - Branch and Faulk were both 2nd-rounders.
I don't buy into the "Manning is a playoff choker" argument - I still have nightmares about the comeback he led in the 2006 AFCCG, plus he utterly destroyed the Pats just two weeks ago - but I do believe Brady has a better GOAT argument once we factor in that Manning spent most of his career in a dome and played few bad weather games and Manning has had better weapons for the bulk of their careers.