I asked the same question, so here's part of an email I got back from the guys over at reefgeek...
"As for comparing the ATI fixtures, the SunPower is obviously the more “economical” of the 2 fixtures. Both the SunPower and the Powermodule feature parabolic Miro-Silver reflectors, identical programmed-start electronic ballasts and an active cooling system. The biggest difference is the housing with the SunPower coming in a more economical bent aluminum housing versus a curved, heavy-duty housing for the Powermodule. The SunPower also has one less fan. Performance wise the SunPower is almost (within 5%), but not quite as good as the Powermodule.
Hope that helps. If you have any additional questions, just let me know. We are here to help!"
Warmest Regards,
Greg Morgan
ReefGeek, Inc.