If I was going to start from scratch with a new system, I would be all over Canon -- Whoops! Did I say that out loud? Are the Nikon police after me???
The only reason I say that is because of camera body performance. But if you are not having to shoot at 8 fps and/or spend more than $4000 for a body -- there is really no difference! Another difference is the option of PC lenses (perspective control), which Canon beats Nikon hands down -- but how many of us are going to spend another few grand to get these???
Optically, if anyone tells you that they can tell the difference, they have succumed to bias driven by marketing or peer pressure.
definitely composition and setting choices are all about the operator, at that is what makes the superior picture, then it is lens construction/choice, then it might be camera type. (at least at this level)
my opinion after a day of shooting... the sensor; pretty f-ing sweet, I cranked that thing to 6400 iso and it was spotless, however id take the build and familiarity of my d200 anyday, there are so many focuzing areas i dont even know where to begin, also their's no compositional focusing which is one of my favorite features, especialy when it comes to sports photo's as i was doing today, my final word, buy a 70-200vr with the money instead, that's the lens i was using and it was soooo amazing, best thing ive ever shot with when it comes to telezooms and this one's vr motor wasn't even working so it even gets better