Just my two cents. I've been really amazed by everything I'm reading about the Cobalt SS. I've never driven either, though. But I can't get that SS out of my mind. It sounds like GM hit a home run with it.
Beside performance, the fuel economy numbers are also no-contest, with the WRX registering 18/25 vs the SS's 22/30. If you are talking STi, it's 17/23. The SS also undercuts on price both the WRX and STi (the STi by a substantial margin). For interiors, I'd say a) who cares in a performace car, and b) you are comparing to a Subaru, not a Merc... Do Subies set some bar for interiors that I'm not aware of?
I assume you have seen this by now:
The Lightning Lap, 2008 - Feature/Features/Classic Cars/High Performance/Hot Lists/Reviews/Car and Driver - Car And Driver