You'd be replacing the D3? If so, then you're giving up a fair amount of high ISO capability with the D3x - I don't have one but it is normally said that it's good up to maybe ISO 1600. As you and I both know, the D3 is good to 6400 without much issue. If you're shooting landscape at base ISO on a tripod, no big deal. If you're shooting architecture in less well lit environments, hand held, you wouldn't find a D3x to be a very good compromise. On the other hand, the D3s is the D3 with one MORE stop of high ISO - and virtually nothing else. If you're not needing MORE high ISO, I'd stick with your D3 - it ain't broke, don't fix it. If you need higher resolution, and can trade off the high ISO and frame rate, the D3x is a suitable match.
If on the other hand you're adding to the D3, I think the D3x is the obvious answer.
The D3s does have power-on dust-shake sensor, which does help some. My impression is that the D3x doesn't have this, and I'm positive that the D3 doesn't.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!