Well, let's start with this first:
My SLR experience is with Minoltas (35mm film) and now a Nikon D300.
I can't give you too many specifics about Canon DSLR's, because I've never owned one or shot with one (other than at a camera store.) I've got nothing against them, and almost bought a 50D recently. The layout of the dials on the Nikons is more similar to what I was used to with Minoltas, where the Canon was quite different with the back dial. That, combined with marginally better weather sealing on the Nikon D300 vs. the Canon 50D, and I decided to go with Nikon. We do get a touch of rain here in Seattle.
Also, I'm left-eye dominant but right handed. That means I'm looking through the viewfinder with my left eye, and squishing my nose against the back of the camera. I noticed on the 50D that my nose was right on the multi way controller. I still hit a control dial on the Nikon, but it is one I can lock out, and it was more comfortable on my schnaz. Maybe the Canon dial was able to be locked out too, but I'm not sure.