The D3x, that I used until the D800 came out, remains a very solid performer with a very good image quality at low ISO (only clearly second to the D800x in my view, close to the D6xo).
It offers amazing physical resilience and a dead solid battery life.
But the D810 is significantly ahead in terms of image quality across the board, AF and functions (EFS, live view, shutter vibration/noise),... Plus it is so much cheaper that you can buy a back up for the same amount.
Cheers,
Bernard