I dont have a D800 only a D700, si I cant judge from own experience, but I frankly couldnt imagine in which regard the Nikon sensor should be better than the S2 sensor...except noise at images over 640ISO.
System wise/Choice of lenses Nikon offers much more for sure if choice means the quantity to choose from.
If someone needs very wide, or very long glass, or zooms in a reasonable size I guess Canon and Nikon offer much more.
But on the other side there is no 50mm Nikon lens which offers the same quality like the 70mm/2.5 S lens (I like 50mm fov a lot),
there is not short tele lens from Nikon like the S-120/2.5 which works so well as a macro but also as a portrait lens.
So I guess it really depends a lot which range of lenses one really needs (which I am sure is very different depending on the photographer).
For my personal use I still own the Nikon for times when I would want to use fast continous AF, or longer Tele. Thats why I keep it but over the last 2 years I didnt really use the Nikon much. It might change when my kids start to do more sports.