D800 vs D3s
Have D800E and D4. They both get used, but for different things. There's not really a lot of difference between the D3S and the D4. Some difference in focus speed and color rendition, but not earth shaking. More of an incremental improvement. The D800E rules in resolution. I use the D4 indoors for high ISO.
The D4 can get full resolution images to ISO 12800 with minimal noise reduction in post. Images up to ISO 50K are possible with significant work on noise reduction in post. ISO 50K-200K is not advised except in real emergencies. You can get usable images at 200K if you could have gotten the image at 50K, but the color balance starts to go out. I haven't had the D800E long enough to really test it at high ISO but I hear it works to 6400 pretty well and you can always downsample and still get pretty good resolution.
The one problem I have with the D800 is that now my computer needs to be upgraded because of the file size. Not so much the normal processing, but when I have to combine several shots (panorama or focus stacking) the computer either bogs down or crashes. The old computer only has 4 Gbytes of RAM and that seems not to be sufficient.
For single shots, no problem.