I get asked which is better, Nikon or Canon, all the time. My response is usually that I prefer Nikon but ultimately they both are great camera systems and it's irrelevant which you have because with either, it's not the camera that matters but the photographer.
This is a fun (although not particularly insightful) chart that outlines where each camera system trumps the other. It's more about which cameras are liked better and by who, and not so much as to the why. Ultimately they basically tie... which proves my point. What are your thoughts on Nikon vs. Canon? Leave some comments - I'd be interested to hear your thoughts. Maybe I'll throw in a few of my specific thoughts as well.