I have and use the D3x but haven't yet experimented the D800, so my answer should be taken with some grains of salt..
I knew at the time I bought the D3x that it would be one of the last cameras without video. That's why I bought it, though many find video on a DSLR useful, for me it's a completely different media.
Then it's also much less ISO capable then the D3s, the D4 or the D600 and D800. I seldom shoot over 1600 ISO myself as for me, photography is about translating what I see, and if it's dark... It's dark
At 100 ISO the colors produced by the captor are quite extraordinary (Raw + different de-rawisers, mainly LR and Aperture). And some users of the D3x and the D800 still prefer the D3x, mainly for that reason. I'll just state that I'm happy with it...
It still has the double slots for the same card format, no hassles with keeping a stock of "classical" cards and finding them on an Istanbul market
The "big" body goes with a "big" battery, about 4000 pictures on one (no fidgeting with the back screen), no on board flash, a second battery and you can stay 10 days in a foreign country without a charger.
The same "big" body can really stand to be showered (it's not waterproof though) and take quite some beating... It's ok with big hands (mines aren't so big but I find the body very comfortable), it doesn't weight as much as it seems in volume (did the test with a D700, same lens and most people couldn't say which was the heaviest !) and though most of my precedent cameras had the "skin" ungluing after two summers (I keep the camera in hand mostly), this one is still in a brand new state.
AF works well and is quick. Menus are just like all the other D3 series, meaning you can jump from one to another without having to change habits
24 MPs are great (I guess those who have a D600 would say the same thing), 36 is maybe an overkill, but then if you are used to a RB6x7 in studio, you might prefer the bigger files
JRP's advice is sound, it's always better to get the "state of art" camera or captor, and in your case, all depend of prices... I had the opportunity to get my D3x new for about 2000$ less then the usual price, about four years ago, so I went for it. If I had to choose today, I guess I would go for a D800 or wait for a D4x of some sort.
As it is, I'm waiting calmly for the next iteration of the "big" bodied Nikon cameras, as I feel I haven't yet reached the full use or capabilities of the one I own... And to be very truthful, I'm saving for lenses more then for a new body...
But then I'm not a "pro", and don't have to argue with clients wanting the "biggest, sharpest, best ever, file"... I just take picture for my own pleasure of discovering the mundane in this world...