He has some great thoughts about both of these cameras after taking them on a trip:
In short: D800: better DR at low ISO; better rez for large prints; but miserable live view implementation
5D3: great all-rounder.
The photos are certainly beautiful, but that fact alone does not necessarily have any direct bearing on the technical quality of the camera.
That the D800 has higher resolution and is good for larger prints, is a given. It hardly needs mentioning, except for the benefit of the complete novice. Likewise with the faster frame rate of the 5D3. These are the most easy-to-understand specs.
However, issues of noise, DR, high-ISO performance, autofocussing speed and accuracy, tracking of moving objects such as birds in flight, and ease of manual focussing when employing Live View, are all matters that should be addressed.
DXO addresses only some of these issues. The fact that the hi-rez LCD screen on the D800 doesn't seem to produce sharp, clear images at high magnification when manually focussing, is a bit of a mystery worth investigating. I'd be surprised if this is a design flaw rather than an initial bug which will eventually be fixed with a firmware update.
If it is a design flaw that can't be fixed, I'm disappointed. I remember well having difficulty being certain I was properly focussed with my Canon 40D in Live View mode. The LCD screen was only 230,000 pixels, and at 10x enlargement the image was a bit fuzzy. This is one of the reasons I was quick to upgrade to the 50D with a 920,000 pixel LCD screen.
The level of detail on the 50D's LCD screen in Live View mode at 10x magnification, with a 400mm lens attached to the camera, is simply amazing.