The Powermdule is available in 8 lamps and 10 lamps, where the Sunpower only goes up to 6 lamps. the Powermodule also has more active cooling inside of it.
We have tested both fixture here in a direct comparison, both 6 x 54w units. We saw the exact same par readings from both fixture at 5 mins, 30 mins, and then at 60 mins of testing to let them get up to full temperature. I would say that if you don't need an 8 or 10 lamp fixture, then stick with the Sunpower and save some money.