No, I'm not going to try to say that either quarterback is more valuable for game-winning drives. That would be insane. Both are probably equally essential when the game is on the line and they need to go win it with a last-minute drive.
I will say that Ben Roethlisberger is more important to the Steelers in these situations because of the simple fact that the Steelers play more of these games than New England does.
If I could pick two guys for a game-winning drive, I'd take either one of these two. They never choke and rarely fail. They're the best two guys in the league. But New England doesn't routinely put Brady in that situation. For a long time, the Patriots simply blew people away.
For Roethlisberger, game-winning situations are routine. Sometimes, as with Jacksonville two weeks ago, he doesn't need to score necessarily, he just needs to keep the team moving and the clock running. Most times, however, he's got to drive the length of the field.
And he's done it. Frequently too. He's won a Super Bowl with a last second drive. He's got more comebacks in the first seven full seasons of a career than anyone else ever.
That's why he's more valuable. The Steelers win more games with Roethlisberger making a late push than they do by blowout or in any other fashion.