The D800 can AF in almost complete darkness, and the D600 is rumored to have an AF system building off the one in the D7000 rather than the D800. It would be reasonable to expect the D800 to have better AF with it's -2.0EV sensitivity and F8 certification.
As for ISO performance, max native ISO on the D800 is higher, suggesting better performance. The D600 will have fewer pixels and a (slightly) newer sensor, so I think best case scenario is to expect them to be roughly equal at 100%, but the D800 would be better when downsized to 24MP.
As you know though, this all speculation until we see actual controlled RAW comparisons. It's unlikely Nikon will allow this camera to have better ISO performance than their $3000 or $6000 cameras, although they do usually put their best/latest stuff even in lower end cameras (D3200).
Mark