They have each taken turns in my opinion. Belichick was the mastermind of the first three titles, Brady took over and won the next two and Belichick had his finger prints all over this most recent one.
I don't think the Pats are close to what they are today without both of them. Considering Brady arguably never would have gotten a chance to play if not for Belichick, it's hard to vote against him, but I voted for Brady.
(The real answer is probably Kraft would was smart enough to hire Belichick and, despite the team's success, stayed out of the way unlike Jerrah in Dallas)