A few years ago, it was Nikon who was behind Canon. Both are really doing quite well. Personally, I'd give a modest nod to Nikon, mainly because they've got a better UI (the cheaper Canon's have a very small grip that doesn't feel very comfortable). In terms of lens quality, it's a toss-up. Some Canon lenses are better than their Nikon equivalents, others are worse. Nikon arguably has a better flash system, but if you're serious about multi-flash coordination, you're probably using third-party gear.

I'd take Nikon or Canon over any of the other D-SLR vendors, mainly due to the wide availability of used gear, rental gear, etc.