I thought we didn't like DXO here
Seriously, I'm sure most of us wouldn't be able to tell the difference between them.
That's probably true (and it's also probably true if you toss the Nikon D600 into the mix as well). And because it's true, and because image quality is ultimately what matters most (well, to some of us, anyway), that says a lot for the 6D. If the images you end up with look essentially the same (though the 6D's may look slightly better at high ISOs and if you go berserk lightening shadows), the question becomes whether the additional features of the 5DIII are as significant as the difference in price; and whether they are depends on what you shoot and your preferences in doing so. They aren't for me - the superiority of the 6D's center focus point over the 5DIII's (not to mention the D600's) in very low light is something I often benefit from, whereas the superiority of the 5DIIIs peripheral focus points isn't something I would notice (and even on the 5DIII the focus points are still lumped together in a rather small proportion of the viewfinder). None of the 5DIII's other advantages matter to me at all. For others, though, the 5DIII would obviously be the better choice (and if I end up deciding I want to use a camera differently, it might well be better for me, too). We're lucky to have both to choose from, especially given the quality of the photos the cheaper one lets you make....