D800 vs D3s
Had D3s, have D800. Only changed because I'm doing architectural work and need resolution.
IMHO the D3s is the better camera. Exposure seems more reliable, Autofocus is less 'fussy'. Better build quality and better high ISO performance (not so much noise as colour and dynamic range... far more useable files above 3200 ISO). If you HAVE to get the shot under time pressure then the D3s will get it. The D800 just seems to get in the way a bit more.
However, if you have time to get the shot then the D800 will produce much better results at lower ISO.
Other benefits of D800 include built in flash that can be used as commander for off camera flash, Auto ISO linked to focal length.
Other benefits of D3s include voice recording for notes and much better frame rates.
The D3s shutter also feels slightly more dampened giving far less of the impression that it will cause camera vibration.