You could see this one coming from a mile away. These 5DIII images have already been posted here. Just change the number in the URL from "01" to "18" to download them all.
http://cpn.canon-europe.com/files/education/technical/inside_canon_eos_5d_mark_iii/01_cinc_big.jpg
Here are some high ISO test images from the D800:
http://mansurovs.com/nikon-d800-high-iso-image-samples
There aren't many direct ISO comparisons (ie 1600 vs. 1600) in these samples, but my own worthless opinion is that the 5DIII at 25,600 looks almost as good as the D800 at 6400, and the 5DIII looks substantially better at 6400 than the D800 at 6400. So, my unscientific conclusion is that the 5DIII tops the D800 by 1.5 to 2 stops.
Discuss
http://cpn.canon-europe.com/files/education/technical/inside_canon_eos_5d_mark_iii/01_cinc_big.jpg
Here are some high ISO test images from the D800:
http://mansurovs.com/nikon-d800-high-iso-image-samples
There aren't many direct ISO comparisons (ie 1600 vs. 1600) in these samples, but my own worthless opinion is that the 5DIII at 25,600 looks almost as good as the D800 at 6400, and the 5DIII looks substantially better at 6400 than the D800 at 6400. So, my unscientific conclusion is that the 5DIII tops the D800 by 1.5 to 2 stops.
Discuss