I just purchased a used D3s, I researched heavily the differances in the two and had the same conclusion that high ISO of the D3s and D4 were very comparable (D3s though much better than D3), and that D4 video is 1080p vs D3s 720p. Video was not a big concern to me as I use GoPro for video and if I want video I would prefer to take my savings from the differance in D4 and D3s and buy a D800 with comparable video to D4 which gives me video and photo capability in separate packages (i.e. shoot video and photos at the same time as I shoot alot of sports). I don't need a $6k video camera. As with most things the correct answer is really what works for you.