Please tell us something about your current situation: what types of photography do you like? Are there any specific features which are very important to you in a camera?
In general, I would say that you're looking at two VERY similar cameras, and so if you know that they have the features you want, your best bet is to try them out and see which one you like to USE the most. For example, I prefer Nikon's menus and interface to Canon's -- but I know people who feel exactly the opposite.
But, if you have specific needs with regards to a camera, that could change my opinion.
- CMOS vs CCD
- 12mp vs 10mp
- liveview vs no liveview (XSi can even display the liveview on your PC. Good for product shooting)
- no issue with all the EF fast primes vs can only get the fast primes with build-in motor