I own and use both D700 and the D800. IMO, the image quailty of the D600/D610 is better than the D700.
However, even with my D800, I find I use my D700 more often than not. Why? Build quality. The D700 is built like a tank, rock solid dependable, and it gets the job done. Also, there are enviroments I do not want to take my D800 into. Rain storms, abandoned buildings, photographing a police stand off once, photogrpahing a fire another time, and even once, downtown Detroit. (Seriously - you wanna take your brand new, pretty looking D610 or D800 there, be my guest.
).
Use the D610 I think you will enjoy it, and there are times you will want and need it, but never get rid of that D700, you will find too many uses for it. When mine wears out, it will go back for a refurb/refit at Nikon service.
good luck