D800 vs D3s
I am lucky enough to have both and I use them for different purposes. In 2011 I went to Alaska to photograph eagles in Haines It was much colder than usual for November, with temperatures down below -20 at times. My D3s and D3x functioned normally, and interestingly, almost all my good shots came from the D3s. High frame rate, rapid and accurate autofocus all worked in my favour.
I did not have my D800 then. For shots where resolution is important, the D800 is king but it is not far ahead of my D3x. There is something I really like about the images from the D3x, but that is another story. For speed and low light, the D3s wins easily, but for resolution, including cropping the D800 is superb.