quite right, the DR advantage disappears at higher ISO when you look at the DXO mark tests.
Also, as pointed out, the DXO mark tests show you achieve better resolution with the Canon despite the smaller Nr of pixels, because each system is a composite of sensor resolution and lens resolution. So the Canon lens is better.
If you need higher pixel density, you can always hook up your 600mm lens to a 70D body
+1 - as you can see in the screenshot below, the combination of the Canon 5DIII + 600/4L IS II delivers better sharpness and less chromatic aberration (CA) than the combination of the Nikon D800 + 600/4G ED VR. (The fact that the Scores are the same despite the Canon combo being sharper and having less CA is a reflection of DxO's Biased Scores - abbreviated as BS for a reason.)
CA is the bane of many Nikon lenses, and their ED elements do not do as good a job of correcting it as Canon's fluorite elements. I like how Nikon claims
(or at least, implies) their ED glass is optically as good as fluorite, and is a better choice because fluorite is so fragile: "Nikon developed ED (Extra-low Dispersion) glass to enable the production of lenses that offer superior sharpness and color correction by minimizing chromatic aberration. Put simply, chromatic aberration is a type of image and color dispersion that occurs when light rays of varying wavelengths pass through optical glass. In the past, correcting this problem for telephoto lenses required special optical elements that offer anomalous dispersion characteristics - specifically calcium fluoride crystals. However, fluorite easily cracks and is sensitive to temperature changes that can adversely affect focusing by altering the lens' refractive index.
" However, they now say
about their new 800/5.6 lens, "...front and second elements are fluorite (a lightweight mono-crystal optical material), which provides superior optical characteristics and reduced weight for balanced handling.
" When they didn't use fluorite, it wasn't any better and was fragile - now, it's 'superior' and they are making a front element from it. Apparently, consistency isn't Nikon's strong suit.
Despite the DxOMark data below, I honestly think that images from either combo will be basically indistinduishable from an IQ standpoint (but it's worth noting that the Canon teleconverters deliver better IQ than their Nikon counterparts). Also, the 5DIII has a faster frame rate and arguably better autofocus, and the Canon 600/4 II is 2.5 lbs lighter than the Nikon 600/4. I can shoot with my 1D X + 600/4L IS II combo handheld, and I couldn't with the Nikon combo. That 2.5 lbs also makes a difference on a long walk/hike. So, based on the frame rate and weight benefits, I'd say the Canon combo is the better choice.