Short answer: Either one.
Both Canon and Nikon have advantages and disadvantages. I personally switched my cameras from Nikon to Canon at the beginning of this year, but for several years I made great pictures, and made money, shooting Nikon.
Things you need to consider are how much $$ do you want to drop into the system. Do you plan on purchasing many lenses for it, and getting the high-end camera bodies, or are you looking for a simple one or two lens system with a D70 body or a Rebel XT body?
I'm a bit more knowledgable about the high end cameras like the Nikon D2x/D2hs, and Canon 1D(s)MK2, so I approach it from that angle. For me, the deciding factors were two things: frames per second at high resolution, and weather sealing.
I need 8 frames per second for my work. I like having 8MP. My previous Nikon system had 4MP, and that worked fine, but 8 gives me cropping freedom, and I can deliver better files to my clients.
The second thing is weather sealing. I was spending a few thousand $$ every year on camera repairs due to water damage. Canon's high-end equipment has weather sealing that Nikon's don't have (or at least, it's not as extensive).
If you're on the lower end of digital SLR's, then either system will work great for you. Try them both out in stores. Read many review sites...not just one. Look at sample images. And keep doing what you're doing here... asking questions!
What models are you comparing, what else do you want to shoot with the camera, and how much do you want to spend?
Mike Isler www.islerphoto.com