It's impossible to say who was more important, but just to play devil's advocate...
The year after Brady left the Patriots, the Patriots missed the playoffs for the first time since 2008, when Brady was injured in game 1 and missed the rest of the season. That year, Brady won the Super Bowl with a team that had missed the playoffs the prior year.
Belichick's track record without Brady has been unremarkable. Both before and after Brady. On one hand you have the best quarterback to ever play the game, and on the other hand you have a coach whose only real accomplishments were when he had the best quarterback to ever play the game...