Since reviewing the photos and getting the wedding blog online, I've got to say I'm very impressed with the 6d.
We go with 2 photographers, one of using a 5D3 now and the other the 6D. The main thing is the focus and number of focussing points on the 5D3 is just superb. I can pretty much take any shot I want now without recomposing, which is also a bit of a necessity when shooting at f1.4 in certain situations.
As for image quality, I genuinely cant tell which is taken with which camera. We will print up to around 30x20 inches on some wedding photos and it really isn't noticeable which camera they are taken with.
Here's a link to the last wedding :
http://kandlphotography.co.uk/keele-hall/laura-dom/
Granted they are only 900px wide but there's no way of telling which was shot with which camera. As always, it's all about the lens. In the above link, it's a 60/40 split between the 5D3 and the 6D.