I don't think it can be taken off Montana's accomplishment; he had to play the AFC team that got there.
But at the same time, Brady had to to this in the free agency era, when it's hard to keep the core of players for a long period of time. Just for that, I think Brady should get the nod.
I think longevity has to be accounted for. Montana won his first Super Bowl in 1981, his last in 1989 (8 years apart). Bradshaw won 4 in a 6 years span. I think Unitas is the one getting the closest to Brady (first in 1958, last in 1970) but by the time he won his last, we wasn't close to the player he was in his prime; in fact, he got injured in the 1st half of Super Bowl 5 and didn't have a huge impact on the game. Brady was even better as a 37 years old than he was as a 24 years old playing his first Super Bowl (last drive aside, he played okay in his first game, but didn't have to carry the team like he had to last night).
Every way you look at it, Brady gets the nod.