The 6D is a slightly-microwaved 5D2 and re-served to us canonites for the next 4 years.
6D does have MINOR IQ improvements over the average 5D2:
- base ISO FPN is improved
- hi ISO is noticeably superior
- center AF point is better in low light
- love that quiet shutter
- intro price isn't horrid
BUT - the midtone fixed pattern noise I found on my 5D2 is still present, if reduced, on the 6D I tested - a minor but there it is, possible issue.
There's likely a few points in the 5D2's favor, like build quality, but I'll leave those to someone who likes
the 5d2 to state.