Upload & Sell: Off
I made a comparison last month between my 5D3+24-70L II and a friend's D800+24-70 AFS.
Here are some screenshots of the images shot at 24mm f/11 to virtually eliminate slight small differences in focussing which could happen at wider apertures, even using LiveView (which incidentally is quite badly implemented in the D800).
Tripod, LiveView, self timer, etc.
NEF and CR2 converted in ACR with zero sharpening and no lens correction.
I then upsampled the Canon file to 36 Mp with bicubic standard and then I applied SmartSharpen (LensBlur) at 200/0,3 for the Nikon and 300/0,3 for the Canon, to partly compensate the upsampling which softens a bit the image.
Nikon on the left, Canon on the right
100% crop - center
100% crop - center 2
100% crop lower
100% crop lower right
100% crop - upper left
100% crop - upper right
Differences at f/11 are small but considering the Canon is upsampled I still rate its IQ slightly higher, more so at wider apertures.
Still the 24-70 G is a wonderful lens and while it is heavier and longer it is amazingly well built.