You're going to get great quality out of either one.  More than anything it's a matter of personal preference  I started off using canon products and never had any problems, so i'm going to stick with them.  I've used Nikon's D300 quite a bit and it's a great camera, but so is Canon's 40D.  When it comes down to it, most people don't even use all the features that many of these cameras offer so unless  you have specific needs, the framing, lighting, ect will make a much bigger difference than the camera itself.