In terms of image quality? Pretty damn close Id say!
I do frequently wonder if I need 36mp... Personally I don't think it's worth it for hobby work. For work work, it gives me piece of mind that I can provide such large files! It is a pleasure to work with huge files for my hobby though, you just feel like there has been little or no compromise or limit on the quality. In fact I almost always feel like the d800 is testing me ... Am I capable of using a camera like this to the extent of the capability of its output??
Would I feel the same with a d600... Not now ive used a d800... But I would imagine I would struggle to tell the difference between the output!
In tens of usability I would also struggle to return to the d600 - it is clearly designed as a step up from an consumer level camera and not a step down from a pro level. It has an "auto" mode and no "AF-on" button, it's small and light weight and feels to me a little off balance with the larger pro lenses (eg 24-70mm) - it does though feel very nice in the hand with the 28mm 1.8 which it's self is very small and light!
tough one really, but at around £1500 the d600 is almost becoming a no brainer (if you have some full frame lenses)