I saw a great youtube video that was shot in three parts by a canadian camera retailer. The first video compared the 5DIII to the D800 in studio shooting and low (as in ridiculously low) light, the second video was comparing them in low light action shooting (rodeo type event), and i didn't watch the third comparison because it was comparing video capability which i don't use. Anyway, it boiled down to both camereas having an advantage over the other in certain scenarios but no clear cut winner. At the end of both of the videos, the guys in it both agreed that a complete brand change would be foolish, as the real world resluts were too close to justify. Bottom line is, nikon shooters have a great camera in the D800, and canon shooters also have a great camera in the 5DIII.