From what I understood, ATI has the best reputation in T5 lighting.
Below are the differences between the two fixtures from ATI's website.
Personally, I prefer the simplicity of the built in controller and single power cord on the dimmable sunpower over the additional 5% par on the powermodule.
http://www.atinorthamerica.com/faq.php
What is the difference between the ATI Powermodule and the ATI SunPower? +/-
There are three main differences between the Powermodule and the SunPower.
Housing Design
The Powermodule has a more of a Heavy-Duty, Curved Housing, much more sleek than that of the SunPower, which has a bent housing. Both are still made of the same high quality German-made aluminum.
Number of Fans
The Powermodule has more fans than the SunPower, but this is only due to the fact that the Powermodule has a more Heavy-Duty Housing. The Powermodule needs the additional fans in order for the Active-Cooling to function adequately.
Style of Reflectors
The Powermodule has a True Parabolic Reflector, while the SunPower has a Bent Reflector, and again, both are still made of the same German-made Reflective Material.
Now even with the above differences, there is only a 5% difference in performance between the two, making the Powermodule more of the showroom and decorative choice, and the SunPower being the more economical choice.