depends what you want to do with it
nikon vs canon debates get old and sour pretty quick; they're both great cameras but nikons' lenses are cheaper in general compared to their canon equivalents. That said, nikon don't have the range of lenses that canon has. Hmm.. But they are significantly cheaper when you get into super tele-zooms and ultra-wide angle lenses. If that's what you want to do, anyway. To use something of that calibre, though, you most certainly wouldn't be using an entry-level camera. So I would suggest investing in the XTI. The XTI does not have spot-metering, however, which is a very big drawback and you have to rely on an external meter to get those trickily exposed shots.
Where do you want to take your photography? Professional? Skip both and get the XSI or the nikon d80; they will take you as far as you need to go as a second body.
Hobbyist? Take the XTI; there's more variety in the cheaper, entry-level lenses.