for non paid amateurs, the 6D2, no question, over the 5D3
for paid pros, the 5D3, no question, over the 6D2
for people that are paid for events, using near identical cameras, a mix of 5D3/4s if you are a canon shooter, with 2 card slots, is a complete no brainer
for the price of the new 6D2, I would think that a used Sony A7Rii could almost be found for that price so I would seriously consider that camera (I own one) - unless you are an Astro shooter
Hockey and wedding photographer. Favourite camera / lens combos: a 1DX II with a Tamron 45 1.8 VC, an A7Rii with a Canon 24-70F2.8L II, and a 5DSR with a Tamron 85 1.8 VC. Every lens I own I strongly recommend [Canon (35Lii, 100L Macro, 24-70F2.8ii, 70-200F2.8ii, 100-400Lii), Tamron (45 1.8, 85 1.8), Sigma 24-105]. If there are better lenses out there let me know because I haven't found them.