If I were a general manager of a team, I would take a great quarterback instead of a great head coach. If you have a terrible QB, your offense won't score. A coach can't score for you, so you need a QB.
A QB can also help coach the offense, and without a good QB you can't win. For instance, the Eagles and Raiders didn't have good quarterbacks and they didn't come close to the playoffs. Those are two examples of why I would rather have a great quarterback over a great head coach.