The cameras seem to be very close. The 6D is perhaps a bit better, with less color noise. If the difference can be measured, I think it would be much less than a full stop.
I installed CaptureOne myself, imported the 5d3/6d raw files I'd say after much more color noise reduction on the 5d3 the difference 5d3->6d is about 1/2 stop - which is more than nothing, given the fuzz that was made about the improvement 5d2->5d3. The real world difference might be that iso 12800 is more usable on the 6d.