Is the D800 better value than the D600?

I don't want to oversimplify but get the one you want. From your post, doesn't sound like cost is an issue. Saying that, I doubt if either D600 or D800 would be a limiting factor in your ability to capture good images.

Like yourself, I am not a pro but have been "keen" for over 30 years. Instead of "downsizing", I upgraded to a "prosumer D300" four years ago when I moved from Pentax system. Except when I need to shoot high ISO, I rarely find an instance where the D300 gets in the way of what I want to do. I don't make huge prints (+2O") so I don't see the need to upgrade just to get more megapixels. To that point, I satisfied my yearning to go full frame by purchasing a used D700 earlier this year. I like the camera a lot (D300 on steroids but I guess that means its a "prosumer full frame" ;-] ) but starting to question the validity of the inevitable need to move to full frame. Sure, there are things that only full frame can do, more megapixels, ability to go ultra wide, more DOF control and...and....hmmm, that's about it. Is that really limiting my photography?

Back to your original question, if you felt the move to a D300 was "downsizing" to "prosumer", I think you would feel going to the D600 would be slumming with an entry level enthusiast body. Yes, that is a little sarcastic but also believe that will be the real rub. At some point, it is no longer about the IQ but how you like to work and interact with the camera as a tool. In that regard, the D800 wins out unless you want to go to a D4 but that is an entirely different discussion.

Cheers!