If you have a 5D Mark III, there wouldn't really be any reason to buy or have a 6D, unless you shot with a backup camera and didn't want to spend the resources to buy another 5D Mark III. Pretty much all the wedding photogs I know shoot with a backup camera, so this makes sense. That's really the only reason I can see in owning both, and I don't think in the real world the 6D offers much anything over the 5D3. However, I can see advantages of the 5D3 over the 6D
So you dont see any benefit in controlling your camera wirelessly or the GPS functionality (yes I know that you could spend a couple hundred dollars on a gps sensor for the 5D). The 6D is said to focus is said to work better in low light, and i have experienced this annoying characteristic in the 5D3.