I wonder how many people, in a "blind" test, could tell the difference (and how many of those who have remarked on the softness of 6D images were looking at images from RAW files).
According to a good German review 6d/5d3 on traumflieger the difference is visible when shooting details like a fern with tiny leaves - and of course they used raw files. But again, nothing very important if not shooting specialized shots like product shots or maybe some architecture.
So what 's your preference 5D3 or 6D after using them?
You didn't ask me, but here's my 2ct anyway :-p after a 3/4 hour side-to-side comparison in a shop just now: The 5d3 is clearly the more professional camera with absolutely superior af - though for some reason or another the 7d grip fits my hand better than the 5d3, 6d is also just fine.
But all things considered, the 5d3 is just like the 6d a typical Canon dslr, I doubt my current shots would improve that much when getting the higher spec'ed camera. So there's no way I'd spend nearly €3000 for a camera body with a soon-to-be-replaced sensor if I'm note having a guaranteed return of invest like a pro would.
And last not least I'm more used to the 60d-style multicontroller and always feel like there's the buttons missing on the wheel - but that would certainly change sooner or later when using a 7d/5d2/5d3-type.
Disclaimer: Of course I would buy a 5d3 for the 6d price or exchange my 70-300L for the 70-200Lii for free.