I think it depends more on the car your talking about. In my Celica the redline is around 7500, and the High RPM Turbo cuts in around 5K. Were as in my AE86 the redline is around 9500K and it's Mid Range Turbo kicks in around 5K. It's also more about the Torque Power Band, and after you install the FI kit, the HP Power Band. If you have a low torque car, or a car whose torque band really gets goin at 4K-5K I would go with the Mid RPM Turbo, or the Supercharger. If you have a good amount of torque, or a car with torque early I would go with the High Range Turbo, or Supercharger.