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The 5D3 AF is amazingly better and you will get shots that you would have missed with the 5Dc or even the 5D2 (even at weddings). I have not tried the 6D, and I am not sure if the 5D3 is worth the extra money that you quoted. However there are other factors to consider especially for weddings. The main ones for me are: the "silent shutter mode", not actually silent but very very quiet, great for weddings!. Also the dual card slots which I am sure you know about. It is a good feeling to come home knowing you have a back-up of your wedding already in the camera. These other 2 factors as well as the AF would push me towards the 5D3. My lenses seems to have a new lease of life as I too have come from shooting weddings with a 5Dc until recently. I can shoot much more confidently at wide open apertures f/1.8 etc and focus on the eyes, knowing that is exactly what will be in focus, with great background bokeh. With the 5Dc I always used to close down to f/3.5 or f/5.6 for safety.