2.25 will improve low end and mid range with some help in the higher RPM's.
2.50 will improve high end and upper mid range and will only slightly hurt your lower RPM range. If you have a 5spd this could work to your benifit depending on your driving style.
If you have an automatic and no forced induction I'd steer clear of 2.5 as it will hurt your off the line power.

 

My cobalt sounded like a V8 for a while. Of course, it had an exhaust leak by the manifold. The thing was disgusting sounding. I was showing my friends at a gas station nearby, when a camaro tried to chyme in with his exhaust. You couldnt her his at all, it was pretty funny.