Bringing the 'different eras and/or systems' into it makes the discussion that much more difficult. It's like asking "Is Sawchuck a better goalie than Roy, since Sawchuck didn't have the luxury of a mask and proper protection?" Physically, most QBs would have a tough time in Unitas' day, but I also think Unitas would have a tough time in the modern NFL's system.
As for Brady having less years than Manning, I'm not referring to simple number stats that accumulate per season, but stats that don't necessarily take number of years into account... Completion percentage, YPA, TD/INT ratio, QB rating, etc.
And I agree that at the beginning of the 01 season, no one was picking the Pats. I didn't mean "ready to win a SB", just ready to be above .500. Compared to the Colts, who, well... there's a reason Manning was the #1. The Colts were terrible.
But right now, you could make an argument for either QB as best current QB. I'm going with Manning, but if someone was to go with Brady, there's only so much you can argue with his success.
And the "Manning in the playoffs" argument is a theory that's just not there, statistically. I'll give you that there might be some sort of intangible next-level thing a QB can do in the playoffs that doesn't show up on the scoresheet, but there's only so far that argument will take you.
Manning in the playoffs: 435/692 (63%) for 5164yds, 29TD's, 18INT (1.6 TD/INT ratio), 7.5ypa, 88 passer rating for his career in the playoffs.
Manning regular season career: 64.5% passer, 2:1 TD:INT ratio, 7.8ypa, 95.5 rating. Considering that the playoffs he is playing elite teams, as opposed to the regular season where he gets very weak defenses to pad stats (like every QB does), that the weather is often colder in the playoffs, his stats aren't really that different. He's 1% less accurate, throws basically the same # of YPA, and the difference i passer rating is negligible, as it comes from the TD/INT ratio.
For a good, quick Manning vs. Brady in the playoffs:
https://www.faniq.com/blog/Peyton-Manning-is-slightly-superior-playoff-QB-than-Tom-Brady-Blog-36420
And for potential "higher than Manning due to playoff heroics":
John Elway playoff - 355/651 (54.5%) for 4964yds, 27TD, 21INT, 7.6 YPA, 79.7 rating
Dan Marino -385/687 (56.0%), 4510yds, 32TD, 24INT, 6.5 YPA, 77,1 rating
Joe Montana - 460/734 (62.7%) for 5772yds, 45TD 21INT, 7.9 YPA, 95.6 rating
Steve Young -292/471 (62.0%) for 3326yds, 20TD, 13INT, 7.0 YPA, 85.8 rating
The only one who stands above is Montana, and I will never argue that Montana wasn't ridiculous in the playoffs. But having the number one ranked player
ever to pass to is a luxury that none of the other candidates came close to having.
edit: jeez, that's a long post. I normally don't like to get into x vs. x or stat nonsense, I guess I had it bottled up.