High ISO JPEG comparison published at fotospekter (slovenian website), In Nikon D600 we can see the noise started from ISO 6400 and rapidly increases as we move forward, the Noise level of Canon 6D is well maintained at ISO 12800.
If we look at previous comparison and this one (above) one thing is clear, the High ISO JPEG Images of Canon 6D is better than Nikon D600. Also see Specification comparison review of Canon 6D vs Nikon D600.