Well, go back a year and it comes down more to brand loyalty than anything else. Compare a 03 Prius with an 04 Civic Hybrid and youll find they are VERY similar cars. A used/pre-owned Prius will be about as much as a base model Civic hybrid, if not a little less expensive. It has almost the exact same rated mileage as the civic, though it actually get better mileage in the city than the highway (52/45 as opposed to teh civics 46/51). The Prius will give you a better warranty, though it doesn't seem to do quite as well in the crash tests. I think that teh Prius looks much better in person than it does in the pictures though.
A reasonably in depth comparison of number between the 03 and 04 Prius, and the 04 Civic Hybrid :
http://autos.msn.com/research/compar...98491&v=t98664