You are using the car for business purpose, so I'll offer two points:
1. Cobalt cost a lot less. Saves you money upfront. Better warranty. You will drive a lot, that 100K powertrain coverage may be a good deal in the long run.
2. Civic will have a higher resale value used. So if you are taking a business expense and depreciation, the car could be worth zero for tax purposes. But sill command a good price used.
As an owner of both cars (Civic in the past), I would say getting 35MPG+ is very realistic. I have not hit 30MPG in my Cobalt (so far). Both auto trans. Mix city/hwy.
2008 Cobalt Sedan LT automatic