The 6D looks like a toy compared to the 5D II.
I own the 5D II and i am a new "professional" and have been using it for 1 1/2 years for over 100 shoots. I always use 2 cameras, and the 5D has ALWAYS been attached to my 24-105mm or 16-35mm. The other camera was a 40D when i started for a few months, to a 1D II for a few months and than a 1D III for the last 4 months. So i have a lot of experience "professionally" with 1D, 40D and 5D.
Yesterday i decided to leave the 5D in the bag and use my old 1D II instead. I must say i loved not shooting with my 5D. I really hate the autofocus of the 5D, especially when you are used to the 1D. It is so easy to miss your focus while using a 5D or xxD compared to the 1D.
Can you really not control the autofocus with a joystick on the 6D? How about using the 8 direction pad? That is the one nice thing about the 5D. The 1D II does not have it but the III does. With the 5D you can quickly go from all focus points to center only or right only with just one thumb stroke, that is a really amazing feature when shooting action of needing to focus on a certain point very quickly, that can mean missing or making a shot.
70% of my shots are used with a single autofocus point, autofocus just doesn't cut it for me on any of them for most situations, but having only 9 little tiny autofocus points can be really tough, but i don't think the 6D will be much better.
Right now i hate my 5D and kind of regret buying it, but will probably keep it for low light and when i need it for full frame, or if i need to possibly record video.
6D could be much better at low light, that is the only reason i would take it over the 5D. The back reminds me of a rebel.. i would really stay away from this.

1D III - 1D II - 5D II - 50D - 40D - EOS M.
16-35mm, 24-105mm, 100-400mm, 40mm 2.8, 50mm 1.4, 85mm 1.8, 135mm 2.0 . 1.4x III, 2x III. 270ex, 270exII, 430ex, 600ex