Although I'm a Nikon user, and have been for many years, I have to say that there really isn't any qualitative difference between Nikon and Canon. They're both first-line cameras with a wide range of bodies and lenses, and most of the second-tier lens manufacturers make lenses for both brands.
My strong recommendation would be to go to a camera store and try both cameras. While Nikon and Canon cameras do the same things and have more or less the same features, each brand has a different way of doing them. You may find one of the brands does things in a way that you find intuitive and easy to use, while I might find exactly the same thing in the other direction. Since both Nikon and Canon are known for consistency, whichever brand you find you prefer you should be able to transition from one camera to another within the line and find the controls are still comfortable.
This is a very personal thing. I've found that I can pick up any Nikon or Fuji camera and know where to find things and how to use them right away - I may not know all of the details of every setting, and I still have to refer to the manual on my D300 from time to time, but I can make the camera work when I need to. I haven't used a Canon much at all, but what little I've seen seems well thought out, if a bit different. I've tried several Olympus digital cameras, and couldn't get any of them to work at all. Their design philosophy is just alien to me.
In the end, most people wind up choosing whichever their friends have, as I did, so that they can borrow lenses and advice as needed.