Difference between Canon Rebel XS and Canon Rebel XSi camera?

I am going on a trip to Australia in a few weeks and I am in need of a camera. I am a beginner when it comes to SLR photography compared to my simple digital camera. Anyways, I have been researching the Canon Rebel XS and Camera Rebel XSi and they are pretty similar but a big price difference. I liked the Canon Rebel XSi because it is good for traveling and for spur of the moment pictures, which is what I will be doing for the whole trip. So should I invest the extra money on the Canon Rebel XSi kit or should I just buy the Canon XS kit?