I've been through the same thing just recently. I was choosing between an XSi (since it's only about $30 more than XTi on Amazon) and Nikon D60 & 80. In the end I just buckled down and ordered an XSi. This is after I've read countless reviews, looked thru countless pictures and thought about every little thing.
I know it's important because most of the time you tend to stick with whichever brand you picked at the beginning. But it is hard because you're not choosing between 2 brands at differnt ends of the spectrum; you're choosing 2 very comparable brands. So I say just go with your instinct. Below are some things that helped my decision.
D80- has focus problems. A few XSi have a similar problem but I think it's just very limited to a small number and I think Canon will replace or fix it. D80 also has older technology.
D60- less MP and fewer features than XSi.
Honestly, there aren't too many negatives about those Nikon models. In fact, I was seriously considering one of them. In the end, I've always liked Canon and wanted the new technology and features so I went with the XSi. I would love a Canon 50D or Nikon D90 but they're both out of my range.