Obviously, for anyone who already has a shedload of Canon lenses, the choice is limited.
But, from serious printed magazine reviews (as opposed to the nonsense you find on the internut), the Nikon D600 is grinding the Canon 6D into the ground and there never has been any competition between the Nikon D800 and the Canon 5D3. However, these things are cyclical. It is only a matter of time (possible just a few months) before Canon leapfrogs Nikon again and, thus, the cycle continues.