There's lots of options. Canon vs. Nikon is the same argument as Apple vs. Microsoft. Buy what works for you. In general, Nikons have always had better AF capabilities while Canon was better on color rendition. I started with Canon film bodies and already had a huge investment in lenses so it was a no brainer when I went to digital. The lens mounts are all the same across all bodies.
One thing to keep in mind, whether it's film or digital, a camera body is nothing more than a light proof box. If you move up the model range, sure you might get a few more features, but the quality of the final file stays pretty constant. The one variable is the glass you put on the end. You're much better off buying a lower end body and using the savings towards a piece of better glass. Kit lenses are less than desirable.
Another option to consider, would something similar to Canon's G12. This is an awesome little camera and comes with enough manual features to keep you interested and learning. Nikon probably has something similar. Head over to dpreview
and do a little digging around.
I ran my own studio for just over 5 years and I'm more than happy to offer any and all knowledge.