I'd always take defense in coaching. QB's like Allen can create offense and there's a number of OC who could score points with Allen. What Belichick would bring is probably the best defensive mind to ever coach and a generally well coached mistake free team. He generally doesn't have a team that fumbles in there own end zone or players that don't know to knock a ball down on 4th and 18 so that the WR doesn't catch it. Team also don't run for 100 yards on him on a regular basis.