We have both at work... actually we have a D70 and a canon 10D but everyone just grabs the 10D because the user interface is easier to navigate.
I used the canon's primarily, but for 1 year, i was stuck with the d70. it's a nice camera, but i still like the ease of use on teh canons'. I had a great lens on the nikon tho - one of them 18-35mm lenses - nice and heavy grade 1500 dollar lens, shoots so clear and fast!
Dont get me wrong, the D70 (or d40) are great cameras... and if you already have a stock of Nikon lenses, then its a no-brainer. But for people new to teh SLR's, i have to say the canon's user friendliness wins.
Go check out a rebel and a nikon. see where u can navigate to change ISO's or set manual features or white balance or just browse the menus in general. see which ones u prefer? for me shooting outside during split second decisions, i need to know where all my camera functions are.
In terms of grip, if you have larger hands, the d40 wins over canon rebels - that is if you are ok with the menu system on nikon. Otherwise, both cameras with good lens setups shoot pretty much the same. some little minor contrasts here and there and saturations blah blah blah...
not to spam the boards with images all the time, but these were taken at an aquarium with real low light. Shot with that F4 lens but with image stabilizer. Those things do wonders...