First of all, if anyone ever tries to tell you that one brand is better than the other, stop listening to that person immediately. The truth is that you can't go wrong with either Nikon or Canon. Both produce equally good camera bodies and lenses.
The cameras you are comparing are in two different categories. The Nikon D80 is aimed at advanced hobbyists, while the Canon Rebel XTi is aimed at the amateurs new to DSLRs and family snapshooters. That's not to say that the XTi is not good enough for advanced hobbyists; it's a great camera. The D80 is more camera and has more features; I know many professionals who use it as their backup camera, and many who use it as their primary camera.
Advanced DSLRs tend to have more dedicated buttons, dials, and switches that make it very convenient for a knowledgable photographer to quickly adjust settings. These extra buttons often frustrate beginners, leading them to give up on DSLRs and go back to point-and-shoots.
Another point that is often overlooked is the feel of the camera. Your average consumer generally looks for smaller electronic devices. Since entry-level cameras such as the XTi and Nikon's D40x are aimed at this market, they are smaller than other DSLRs. If you have large hands, you may find the XTi is too small for you. Take a trip to your local
Best Buy and hold the cameras, see which one feels better in your hand. Of the two cameras you're looking at, the D80 "feels" better in my hands.