It looks like you're new here. If you want to get involved, click one of these buttons!
I've seen many used D3's show up at a local camera dealer. They usual have very low shutter counts (less than 50,000), most likely used by amateurs or as a backup. If anything pros take better care of their camera bodies, taking them in for service more often to insure that they are functional.
So at $3,000, would you rather have a D3s with 50,000 clicks or a new D800 if you were shooting landscape/wildlife/architecture, etc.
but kind of heavy
D800 vs D3s @6400? My guess the D3s would have cleaner files. D800 more detail. That is a point when you have to start weighing how much cropping you do. If a lot, D800, if none, D3s.
. I think the difference between the D80 and the D700 is far greater than between the D700 and D800. Only downside: it's less of a long term investment that way.
That's a tough one. For wildlife the D3s is better, simply because it is a faster camera in many respects. For landscape and architecture the D800 would be the better choice. I think it would be prudent to pick based on what you do the most out of those two categories.