Personally, I would choose the Canon Rebel XSi unless you had some extra money that didn't know what to do with. And I have some good reasons. Canon has always been at the top of the digital camera market and in particular at the top of the market (along with the Nikon) REFLEX. Your Digital Single Lens Reflex cameras are all excellent. It could not make a mistake when you purchase any of them.
Even the lowest line of Canon , the Canon rebel DSLR, it is very well classified by all the camera review sites. A serious while there are a lot of really good to choose various Canoncameras, not every one of them is a great option, and you would have to do some research before making their purchase decision, unless you had some inside information of a friend professional photographer. But buying a Canon DSLR is a no-brainer. They are all good. You will be truly impressed with the quality of the images.
OK, well can you choose the Rebel XSi on the XSi? It is due to the prices. The XSi is more than $150 cheaper it's younger cousin, the XSI. And for the beginner photographer (which is what most of us is when we are buying our first entry-level DSLR) differences in technology of the two cameras are virtually non-existent. The exception is the high definition video. If you have video digital camera, should get a Canon Rebel XSi. Otherwise, go with the XSi.
Use the $150 save to buy an extra lens or tripod.
Bottom line, if you're wondering the question "what rebel Canon should I buy"... You do not make a mistake, because both are fantastic. Buy an Canon Rebel XSi or XSi will transform your life photo, especially if you have this lens extra. Make a macro or a zoom to give you a little more than flexibility.
You might want to take a look of real for each one of these cameras specifications. There is a great comparison in http://www.digital-photographic-resources.com.