I won't even attempt to advise you on the "value" of one over the other, but I recently pickedup up a used D3s, adding it to my D700. I shoot a lot of high school sports under outside lights.
In my experience, I've found the images from the D3s at ISO 12,800 to be of noticably (though not dramatically) better quality than those from the D700 at ISO 6400. Mind you, the D700 at 6400 is still very usable and prior to using the D3s, I was very impressed. But even at 12,800 the images from the D3s are not only showing less noise, they also seem to hold truer colors and more natural/realistic contrast.
Only you can answer the value question, but there is a definite improvement with the D3s over D700. Can't really speak to the D800 at all.