go test drive a cobalt and an aveo, they handle VERY different from each other. I personally think the aveo feels much better (more sunfire like?) and the cobalt will leave you disappointed. So get that out of the way first.

The hatch has the advantage of the fold down seat. Which is only an advantage if you have 2 or possibly 3 people in the car. You can stack things higher in the hatch, and with the seat down it has tons of storage. Even possibly enough room to sleep in..
I can't speak badly of the sedan aveo in this aspect, but a coupe would be harder to fold down the back seat to load fully.

The wheels and tires will not fit. tires are too big.

I do not know the slave cylinder issue you have concerns about, but the slave cylinder bolts to the side of the transmission.