I use both the 5D3 and 6D extensively and about equally for weddings. I'm very happy with both. The 5D3 is better overall -- it is superb, especially the AF -- but also heavier & costlier. The 6D's center point is a bit better for very low light AF, and I appreciate the lightness of the camera. Image quality is pretty much equal. You really can't go wrong with either, unless you specifically need what one has and the other doesn't have, such as dual cards in the 5D3 or wi-fi in the 6D.
I could say exactly what Zlatko has said here the same for me.
I would add that the 7D will quickly fall out of favor for you should you pair it with either of these cameras. The image quality will be lacking, and you'll not want to use the 7D soon after pairing it with either the 6D or 5Dmk3. I got the 5Dmk3 and paired it with the 7D, but I found I couldn't use the 7D in place of the 5D3 ever, so there was no point to keeping both. If you can afford it, get the 5D3, sell your 7D and use that to pay for much of the cost of a 6D. These two together make a nice pair. The 5D3 has many great features and will be the more versatile camera. The 6D will prove to be a very handy 2nd body, especially when you need the better low light AF and want to go a little lighter in weight. I can use either of these cameras in almost any situations, although there are some situations where one is clearly preferred. The 7D just doesn't have the image quality to ever be used instead of either of these cameras. Once you get one of these, you'll be looking to ditch the 7D as soon as you can.