I think a quarterback is more valuable than a head coach. A good quarterback should be a leader and set the tone for an entire team. He is one of the most important players on the team and must coordinate with other coaches.
Quarterbacks can make changes to a play at the line or improvise when a play breaks down. The head coach does a lot of things from the strategy side that a quarterback would not do. A quarterback in the NFL should be focused soley on the game and not worry about head coaching duties.
A good head coach and a good quarterback are necessary to win in the NFL. They both do different jobs for the same team but I think a quarterback is more important.