The d610 sensor is really nice. You would notice the difference in terms of general resolution and shadow recovery. Probably a little cleaner at high ISO too.
I quite liked the portability of the d610 when out and about with family or walking about in the hills.
And the last advantage for d610 is the dual card slot which alleviated some paranoia.
Everything else I can think of, the d700 is better. Better feel, better control layout, better af system (much better in fact) etc. I also love the viewfinder curtain for what I do. Missed that on d610.
So, depends what you'll use it for.