The reasons for me to choose the D600 over the D800 are very simple:
1. It does what I need it to do. 2. Cost.
For me, the D600 is at the "sweet spot" for price vs features. Of course that will be different for others, and that is why Nikon makes a wide range of DSLRs. I see no need to pay for a D800 when the D600 does everything I need.