All the discussion of which QB is better based on which criteria compels me to say something many will consider sacrilege.
Joe Montana was a good QB in a great system and situation.
Football is the ultimate team sport, yet people still insist on evaluating QBs primarily on team accomplishments.
Here is the question we all should be asking in these discussions, if you switched team and coaching situations between two QBs would the performance be markedly different?
Montana was great with the 49ers, and Bill Walsh and a good defense. Put him on the Chiefs and he was still good, but not near the same performance as his stint with the 49ers. Conversely, the 49ers didn't miss a beat without Montana. Young picked up right where he left off. Although a key component of the 49ers success for years, Montana was far from irreplaceable.
I think Brady is in the same boat but we haven't seen Brady outside the vaunted Patriots system.
My feeling is that most of the top QBs in the NFL could swap with Brady, once mastering the Patriot system, and have nearly the same success. And I think the same probably holds for Montana. Walsh could have taken those teams to the same success with a number of good QBs to execute his system.
Team accomplishments should be evaluated as just that, and only rare individuals who demonstrate success across different teams should be given credit for unique contribution to the team.
Brady and Montana are both good, great if you must. But they also got lucky to fall into favorable situations to maximize their skills. Give those same coaches the same team, but switch with another top QB, Marino, Manning, Brees, etc. and I am betting they don't miss a beat in team accomplishments.