If you took both the 5dmk3 and the d800 along with 2 flagship lens for an outing or event - or lens of choice as long as its similar to each manufacturer, i.e. canon 50mm L and the nikon equivalent. Or the new canon (when avl) 24-70mm vs, the nikon counterpart.:
1. Would you load the images and be wowed "OMG the d800 is better on paper and my images truly reflect it! im so happy..."
2. Find flaws in images from both cameras and observe where it was technically the shooter not the lens or body? while some acceptable variations may occur the overall results compared would be similar.
3. Or make a choice not bashing one or the other resulting in proffering the ergonomics of one, the processing, and all overall basic results.
For example on #3, when I shot film there was something that I favored in nikon film cameras, the color was different, i favored it more. I shot the counterpart canon body and lens and preferred the results form my nikon film camera, it was to my needs. Now canon fills the need, or i can adjust the need in pp.