Well, I was lucky to find a used Df with someone that just needed money to pay for tuition and ready to take my D600 in exchange. Otherwise I'd have stayed with the D600 and would probably be very happy with it. I can't say the difference would be worth the $ difference if bought new. I was just seeing the value of my D600 go down with the arrival of the 610 and 750. I'd say that if you could get your hands on a D600 for around 1100.00 $, you'd be very happy with it. Newer models always have something the older ones don't, but at a price. It's when you consider the price difference and you realize that you could use another lens more than a newer body that you think twice.
So I'll repeat what I've been told and have known for a long time: Invest in lenses and use whatever body you have until you have all the lenses you want.