It's my understanding that Ithaca Carshare will be subsidized by the University. I would just go for that:
Cornell Chronicle: Ithaca Carshare launches
Getting to campus from the Commons is pretty easy. Busses basically run every 10 to 15 minutes during the weekday, and are generally very punctual. Getting to the Pyramid Mall isn't too bad either. Getting to Wegmans by bus will be a little bit more difficult, but it's a very easy bike ride if you are already living down by the Commons.
I would recommend a car if you like the outdoors and are interested in exploring Upstate New York on the weekends. I did a lot of snowshoeing and hiking in my days at Cornell, and it was much easier with a car.
Also, what will you be studying at Cornell? If you are at one of the professional schools or in the humanities, a car would be less needed than if you were in the natural or physical sciences and were working in a lab all night long. The walk down to the Commons feels much longer at 2AM than it is at 6PM.