The 6D is a microwaved 5D2, which was already a re-cooked 5D, which had its original serving as a 20D. (In terms of actual camera performance, not IQ )
If you want a better performing camera, 5D3 is the best route. It won't limit you later on like the 6D could with its AF performance.
One question: are you making this judgment after having used both cameras, or just because you think this is true?
I can tell you from having used a MKII professionally extensively that in many ways the 6D is a big step upwards from the MKII. I have not owned or used a MKIII, although I have extensively researched it. The MKIII is without the superior camera overall, but even that is not straightforward. There is some give and take with both bodies, and, for me, I concluded that the "give" with the 6D right now was worth the "take".
The truth of the matter is that the 6D actually has more modern technology than any other Canon camera at the moment, even if the form factor and AF are not premium.
Yes, I used the 6D for a day rental and found it's AF nothing particular special from my previous 5Dc or the MK2. I didn't have time to really test it's reported -3EV center point AF, but I could imagine its spectacular in low light as the 5Dc and MK2's we're pretty good.
As for the other features, They're nice fluff but I know it's a camera that could be outgrown if any serious AF situations arrive. IE: off-centered subject approaching camera while shooting Sub-F/2.8.
Perhaps, the 7D spoiled me with these expectations but the MK3 is a camera I will not outgrow for awhile.