Superchargers are like 35% parasitic on an engine whereas a turbo is like 0% parasitic. I read that on these forums somewhere back a few years ago.
Because of the extra energy the engine has to use to turn the supercharger through the belts and everything, it is parasitic. A turbo, which is spun by the exhaust gasses, is not parasitic. Thats why at the same boost level, a turbo will help produce more power