Having owned neither but used both i would say both lovely cameras. If it was purely down to IQ the sony made sensors as previously said seem to be well on top at the moment. This would point at DxO being right in their mark. t the end of the day it is normal for someone to defend their choice of equipment and to be fair both these models are top class in their price range.
I did find it amusing earlier though when it was said the 5D af is better (due its 61 points etc) but the image quality from the sensor is different. I dont particularly like DxO and maintain what i said earlier about in real life images it would be hard to see a difference,BUT in tests the d600 is better in IQ. If you only need a "very good" af system (and the af in the d600 is excellent, no 2 ways about it) and are not bound by lenses the d600 would probably be a more sensible choice, however if you need the superior af of the 5d or have lenses then it is a different story. I would be happy with either, both would work for me