The Belichick "vs." Brady debate is a "hypothetical" wrapped up in a "possibility" and surrounded by a "maybe." It reminds me of the principle in Physics that posits that there are things in the universe that you can't observe without changing them, so you are left with trying to theorize what "they" really look like and how "they" really behave.
it's already been observed that most great coaches have been, at some point in their careers, joined at the hip with a great QB. Brown/Graham. Lombardi/Starr. Landry/Staubach. Noll/Bradshaw. Walsh/Montana. Shula/B. Griese. Belichick/Brady. Grant/Tarkenton and Levy/Kelly (the latter two w/o rings).
But, Bill Parcells went to three SB's, winning two, with three different QB's (Simms, Hostetler and Bledsoe). Gibbs went to four SB's, winning three, with three different QB's as well (Theismann, Williams and Rypien).
I think that this is so close a call that you can argue it either way.
The simple fact is that we don't know what Bill Belichick would have done between 2001 and 2007 without Tom Brady. Like the physicist trying to observe sub-atomic particles, we can only theorize. The same can be said of Brady. We just can't know how he would have performed without Belichick's system. Once again, we can hypothesize and argue; but, we can't know.
If forced to do so, I guess I'd come down on the side of those who suggest that the scales are tipped slightly (very slightly) towards Belichick in assigning credit for the Pats success since 2001. Their success this year (and I'm calling 11--5 "success" with or without a trip to the playoffs) with a QB who hadn't started a game since High School supports this perspective. The argument that BB wasn't as successful in Cleveland without a Tom Brady overlooks that it was his first job as an HC and that he was in an almost impossible position with Modell and Kosar. But, by the same token, if you asked me would there be a Bill Belichick without Tom Brady, I'd answer, I don't think so; conversely, we can only speculate as to whether Brady would have succeeded as wildly without Belichick.