...and once you've invested in one...
Yes that is the crux of the matter in terms of the nikon vs canon (or whetever else) debate. Once you start buying lenses, you stick with the brand you started with.
I have remained canon because I went there at some point in the past (my first canon SLR was a 35 mm and the lenses fit my current models), and the next time i wanted to upgrade a body well...
I once had someone tell me he found the modus operandi of nikon "more intuitive" than that of canon. Well yeah....if you are already familiar with nikon. If you are familiar with canon, then you find its modus operandi more "intuitive" than that of nikon ! Where do these people come from? I'm the bloke that doesnt find the modus operandi of an i-phone intuitive...until I start to become familiar with it...ie I dont find anything intuitive from a "cold start".
I currently have a base model DSLR crop model canon as well as a full frame canon. The difference is in the operator
although the full frame is better set out in its controls (bigger body - more room?) and thus easier to use. The crop body suffers from having to double up on some functions which makes it less convenient / slower when you are in a hurry. But the photos are great and in fact looking at all this laterally, rather than to say that all the pro / full frame gear is so fantastic, I reckon that the base model DSLR's (both brands) really are fantastic for what they can give you in a very budget price compared to the cost of the "good" gear.
Now if canon could just see their way to fitting a second wheel onto their smaller /budget end crop bodies, they would go a long way to improving their operation.
Err...not really trying to hi jack the nikon thread, but I think the comments are relevant enough.