Canon and Nikon both make great products. I use Nikon, but Canon is good too.
I don't have much experience with the Canon products. Have used friends' Rebel and 30D, and they're OK.
I have a Nikon D2H and my wife uses a D70s. The D70 is a great camera, but the viewfinder/mirror is narrow. If you've no experience with a "flagship" DSLR like the D2H (or a Canon 5D), you'll never notice the difference. The D200 is a much better camera than the D70. I swear by Nikon. Have used them for 30 years.
If you're at a point where you have no lenses for either brand, you're in a great spot. The following reflects my opinon only.
Canon: Superb lenses, fair menu access and layout, fair bodies, great imaging hardware inside the camera, less than desirable ergonomics.
Nikon: Great lenses if you don't buy a G-series lens, good menu access and layout, great bodies, superb ergonomics, I'm disappointed that nikon hasn't or isn't going to go full frame on their imaging sensors ever, but the sensors they are using are great.
If you're talking to a camera shop (a real shop, not frickin' Comp USA or Best Buy) See if they will let you play with both the Nikon and the Canon units. Judge for yourself. A word of warning. The "this one has x-zillion megapixels" is useless for the most part. Pixel quality beats quantity hands-down every time. My 4.3 MP D2H looks better than the 8mp D200 and the 12mp D2X, and I can shoot at 8 frames per sec until the card is full. Don't believe the hype from the sales guys.
Kev