By DxOMark, the most trusted sensor performance measurement institution. Don't comment with fanboy angry words, this is a factual lab measurement that leaves room for little comments:
|Canon EOS 5D Mark III|
Not only that is a huge difference, but since Nikon is beating Canon at everything single measurement, even low light performance, that is humiliating. FYI, the Nikon D800 is the highest ever at that benchmark.
The Canon is pricier and supposed to perform better in low light...well not even there. And you don't even have the huge definition...ouch.
With digital, dynamic range is traditionally very poor (that's why I rarely shoot digital), and the Nikon is doing ok from ISO 100 to 400. The Canon EOS 5D Mark III doesn't. The relatively poor built quality of Canon VS Nikon is a well establish fact (take both in your hands and be honest with yourself), and since the 2 most important things for a good picture after the subject and the photographer are the lens and the sensor, we are left with one question: is Canon at least better at lenses?
In regards to the amount of pros I know who shoot Nikon lenses on Canon with an adaptor, and the number of poor comments of premium Canon lenses (some like the 17-40mm are beaton by Tamrons). I'd said the conclusion is pretty clear: Canon has by far the best marketing department.
Now once again, you can be an awesome photographer with Canon gear, as well as with 40 yo gear. The camera doesn't do much, but the latest Canon I used (a 7D) was highly disappointing, the 2500€ lens I had with it was just has bad. I am more and more certain that for the time being, Nikon VS Canon is a no brainer. It has been the case for as far as I can remember actually (my F5 was already nailing competition in 1995). It just is unfortunately virtually impossible to get a Nikon D800.
Then of course if you like the real stuff, you don't care since you shoot Velvia or Tri-X on a medium format camera :D.