Can I ask about the 6D sharpness compared to the 5D3? Is it just the slightly higher pixel density of the 5D3 or do you think there is another factor in play?
You can discover this yourself by downloading any raw files with fine details yourself, use the dpreview widget http://www.dpreview.com/previews/canon-eos-6d/11 or look at this comparison: http://www.etherpilot.com/photo/test/misc/6d_5d3_d600.jpg
The difference is not big enough to make a difference for me, but it is visible. The best test I know concerning the resolution differences is on the German site traumflieger.de - look for the 6d review video.
From what I've read and seen for myself this is not only due to the lower resolution of the 6d, but because Canon applies more forced noise reduction to the 6d raw files - look at the strong chroma noise on the 5d3 in black areas, it's nearly completely gone on the 6d but at a loss of sharpness.
To put it another way: If you downsize the 5d3 files to 6d resolution and apply strong chroma and mild luma nr until the sharpness matches the 6d it's a wash except for a slight luma noise advantage of the 6d @same iso: http://www.canonrumors.com/forum/index.php?topic=11279.msg202164#msg202164