Do you own these cameras or are you quoting the internet?
You see most of my equipment under to the left - and I already stated that the dr comparison is from the traumflieger review. Also see here, the 6d has ~1 more stop of dr at higher iso: http://www.sensorgen.info/ ... though of course I cannot verify the validity of their testing and methods.
5d2 - 11.2 dr
5d3 - 11.0 dr (like 6d @iso 800)
6d - 11.5 dr
5d2 - 8.3 dr
5d3 - 8.5 dr
6d - 9.3 dr
The said review is very definite on the spatial resolution test, and at least this falls in line with what the CR user mst (Canon beta tester) often wrote and what I've seen myself when looking at 5d2/5d3 sample raw shots.
when i first got my 5dmk3 and compared it to the mk2 i was super disappointed because i was only looking at pure unedited raw files
the 5d2 files were sharper when pixel peeping however after having used the 2 camera side by side for over a year now i have to say absolutely the 5Dmk3 files respond much better to post processing they can be sharpened more without artifacts occuring the files respond much better to noise reduction at any iso as the noise is cleaner and less blocky, it has a more organic film grain feel and as I said there is definately more recoverable highlight headroom when exposing to the right.
Just be carefull quoting the internet categorically without actually having first hand experience
the 5dmk2 still makes nice images for sure but the 5Dmk3 is better accross the board in every single respect
except maybe the silly picture review zoom
I put less than zero credability into any of these back of the lens cap naval gazing DR tests people seem obsessed about.
I'm talking about real world shooting real world processing and real world results.
i havent used a 6D so i wont comment on its real world abilities however the camera doesnt really meet my needs and the 5Dmk3 far exceeds it in a significant number of areas that are significant to me