The Cobalt SS is the main competitor for the Honda Civic Si so my suggestion to you is consider, do you want the Honda or the Chevy...
The SS is by far faster than the Si has more horses and better handling but the Si is in my opinion better looking inside and out, the performance in still incredible but not quiet as fast on account of the turbo. As far as fuel-economy is concerned they both get the same amazing economy.
So my advise to you is test-drive both cars, make sure you get them to the red-line at least once (the sound of a turbo is sometimes unappealing) and base your decision on that.
Hope it was helpful!