For some things, the Canon XSi is a very god choice, but the Sony has some advantages. And in some instances, the types of things you want to shoot make a difference.
To cover your situation in a few broad strokes:
- For dSLR bodies, "New" is always better than "Used".
- Canon has a better selection of OEM and third party lenses.
- Sony has image stabilization in the camera body, so every OEM, third party, and used lens will be stabilized
- Canon relies on image stabilization in certain lenses, which makes them bigger, heavier, more expensive, and more rare than lenses for the Sony.
Based on the limited amount of info you've provided, between the two, I'd go with the Sony.