Canon EOS 5D Mark III. Panasonic Lumix DC-S1R. 81. 100. Compare. Canon EOS 5D Mark III vs Panasonic Lumix DC-S1R.
Video -> 6D, only photo-> 5D3. The autofocus system on the 6D might be newer but it's worse. It's the same AF system as the 80D, but on a full frame. Same size, so it doesn't go all the way to the edge of the frame. If you only shoot personal, I'd go for the 6D regardless, if you're pro, then 5D
The shutter life rating on the 6D’s shutter is 100K actuations. The 5D3 is rated for 150K. If you are the kind of user who is likely to hit the higher shutter count during your use of the camera, this is definitely a consideration. That being said, replacing a shutter is around $300; it’s not the price of a new camera. The 6D would be an upgrade from the 5D, even compared to the 5D MK II. In low light noise the 6D even has an edge over the 5D MK III, but it is really splitting hairs, but the 6D features are less than the 5D MK III and the 6D with only one SD memory card slot and requires using the menu system to change white balance, so not as easy with buttons to quickly access some features.