I just sold my 5D2 and replaced it with a 6D last month. I can without hesitation say that the 6D is hands down a better camera than the 5D2 in every way except price.
It's smaller, lighter, has silent shutter, useable 12800 ISO, better controls (lack of joystick aside), similar or better build quality, similar or better image quality at low ISO, far superior image quality at high ISO, superior AF performance (especially the center point in low light) and the list goes on.
I would never go back to a 5D2. I would, however, go up to a 5D3
EDIT: Oh, and I forgot to mention the Wi-fi and GPS. Two features I don't really care about but they are actually pretty awesome if you plan on using them!