Cobalt is faster and better performance stock. SI is a blast to drive and with a little tweaking you can get some decent performance out of it as well. I went with a better exhaust, and a cold air which really helped on the low end torque. Resale is much better on the Si like the other guy said. And you can really extend the gas mileage if you dont drive it hard. i can get close to 350 miles to a tank if i granny shift it.Only thing is that I do not like about the Cobalt SS is that ridiculous wing that is on the back. I think it is rear wheel drive though, which i would like better than the FWD. All in all i love my SI. I test drove the Cobalt and i jsut didnt like the interior styling.