In order to get power out of the exhaust there must be a pressure/temperature drop between the two sides of the generator (be it a turbine or roots type charger). I would stick to a turbo type setup since it is designed to do what you want. A roots type charger really is not designed to take power out of something, its only designed as a compressor. I am assuming you mean a roots charger like this:
Roots-type supercharger - Wikipedia, the free encyclopedia