The 6D is a bit better than 5D3 at 12,800 in terms of noise. After manipulating in Capture One 7 I was able to get them very close however. The 6D shots have a skew towards red visible in the third scene and the 5D3 color balance and renditions look better to me. That is viewing in both DPP and Capture One.
JMO, YMMV, etc...