canon: Better color saturation, slightly less photo noise, cheaper lenses
nikon: Sharper lenses, more range on ISO for better fast-action or low light.
Pick your camp and defend it like a religion.
I personally have been shooting with a canon XSi for about 18 mo and love it. If I had to do it again, though, I'd go up one model to get the HD video capability just for play value.
I'll strongly second the idea that you need to spend for the best SD chip on the market. Cheaper cards are slower so you'll never get the frames per second the camera is capable of. And that's important shooting autosports. Also, read and reread the manual. Honestly, studying the manual that came with my Canon really improved my photography
See my Smugmug site for samples from my camera, mostly with the cheapo zoom lense it came with.
"It's hard to be a visionary in a world full of blind people." - JM