Go to many stores and try them all out. Best buy usually has a d40 as well as Rebels on display. The only complaint I really have is I need faster glass for taking action shots. This isn't the camera's fault just the lens I have. What I do to compensate for this slowness is predict where my target will be.
I'm a biased rebel user but even with that said Rebels a great camera that can do a lot.
Being a new camera owner (Rebel XT), what I have found is that the body is just about the cheapest part of the purchase. I think both the Nikon's and Canon's will probably net you the same quality photo as long as you have comparible lenses. I bought the XT because my friends had recommended it to me. I love my camera and couldn't imagine owning anything else.