The head (weighting, loft, face angle) has the biggest influence on the ballistics of the ball. The shaft (bend profile, weight, flex) has the biggest influence on how you swing the head. It takes getting both right to have a good driver, but the right loft is absolutely essential in getting an optimized driver. Based on your swing speed, you have an optimal launch angle that will allow you to reach your potential. A general rule is to choose the least amount of head loft that will let you hit that optimal launch angle. Once you have the "right" loft, you can fine tune with the "right" shaft for you.