I have the 5DIII and have used the 6D.
The only thing I prefer about the 6D is the weight and size. The 5DIII has better ergonomics, better button implementation, better menu structure, better HDR implementation (RAWs + HDR saved), better AF, far superior video performance, ... basically better everything.
Not better everything, but I'll concede most of what you mention. I prefer the ergonomics of the 6D, myself. It could use a few more buttons, but definitely the shutter button has a better feel on the 6D, than the 5D3 to me...it's mushy and vague.
However, again...the 6D is now half the cost of a 5D3. The sheer fact that they're mentioned in the same sentence, says a lot about what you get for the money spent on a 6D.
I agree the 5DIII shutter button feel could be better. I found it odd at first but I guess I've gotten used to it. No question the 6D can take every bit as good a photo in many situations, especially portrait and landscape. But I use my camera as a general purpose everything machine, including some sports and video.
The 6D video performance is the fatal flaw for me. I hated the moire and aliasing on the 5DII. The 6D performs about the same as the 5DII in video, but on the 5DIII Moire is almost completely absent, and that is a big deal! There's also audio in and headphones out and audio levels while recording, soft shutter, silent mode for video, and quite a few other nice touches.
If I only wanted landscape and portraits I would get the 6D and save some money for sure. But the 5DIII is a pretty awesome general purpose tool, with no major shortcomings.