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...get a D4. Prices will soon drop as it will be two generations old.
Not really sure how to answer this one..... I have a D700 and a D810..... The D700 sits on the shelf... Next to the D70.For me it would not make sense to get a D3s. If you haven't gone full frame yet, a D3s would be nice, but then I would have to argue that with the right glass, the sensor and noise in the D610 would out weigh anything the D3s can do, unless you need a very specific feature that the D3s has.
The advantage for me of having a D3s, or a second D4 is to carry both with primes and be able to switch with the functions in identical positions. And, this may be more because as I get older I find it much more to my preference to have buttons, menus, etc., always in the same spot vs. significantly different camera bodies. My D90 has not been used for a couple of years at least. About time to give it to my grandson.......and grab another pro body.....LOL