I shoot with both. For general use, landscape, street, etc. The D800 is wonderful — and lighter to boot. I've also shot some (poorly lit) off-off-Broadway theatre, auto ISO floating between ISO 2400 – ISO 6400.
Perrone is probably right: at ISO 3200 there's no difference in the noise; at ISO 6400 the edge may be to the D3s, though a perfectly exposed shot ISO 6400 almost matches the D3s (of course, when stretching to ISO 6400 I'm not sure that perfect exposure is truly obtainable ).
For those times when I shoot high FPS, the 4 FPS of the D800 is usually fine. It flunks with baseball (II haven';t shot much sports since college and don't have Perrone's skill set and experience) and BIF are a bit iffier than with the D3s. Of course, BIF are iffy with any body…
If FPS doesn't matter, and high ISO use is rare and tops out at ISO 3200, then I'm hard pressed to think of why you wouldn't want a lighter body with more DR and lot of droppable pixels!
Jon Kandel A New York City Nikonian and Team Member Please visit my website and critique the images!