Whatever one feels better to you in your hand, that you like the way the lens zooms and/or focuses. If you can hold off a month, rumor has it the D90 will be announced in June, so D80 prices may come down if you are interested in that.
But one odd thing that came up is that Canon switched the Rebel series to SD cards from CF like the others, so both of those that you're looking at use the same memory card.
Nikon's D40 and D60 can only use the more recent Nikon lenses to autofocus (has a built in lens motor). i.e. the Nikon 50/1.4D will not autofocus on one of those. The Rebel will work with any AF EOS EF lens. But that is why the third-party manufacturers have been updating their own lineup to add a motor. Sigma's new 50/1.4 will work with the D40/60, and Tamron just updated two lenses with motors as well.
Did you look at the Sony offerings?