I have both.

Castle system.... seems "faster." By that I mean it winds out more. The controller is not as smooth, it is more like a light switch.

Viper system.....Is smoother, love the progauge for adjustability, the top end seems flat with recommended gearing. I geared up and it screams now. Has good torque, but doesn't feel as strong because it is smoother than the Castle.

Another friend who races club races is switching to the copperhead from the sct/3800 combo, too.