The 6x54 watt ATI PM is cheaper than the Constellation, the 8x54 watt about $125 more. The ATI is far quieter, performs better, and looks alot more high end.
I have not used a Constellation, but have seen 2 in person- they are better than most fixtures out there. But with the comparable price- you cannot beat an ATI. You could consider one of the Sunpowers and save a few hundred dollars- about the same performance as the Powermodule, just not as fancy.