I have both the D800 and the D3X and while I use both bodies I find I'm using the D800 more. The advantages of the D800 that Ramon points out are very valid. However when I don't need to heavily crop or shoot above ISO 6400 there is something about the D3X photos that I prefer. It's hard to pinpoint. To me the photos from the D3X look more refined and more pleasing. Also the high ISO performance of the D3X is quite often understated. I shoot at ISO 1600 at least half the time and don't see noise unless I go up to 100%. The D800 also appears to be a more finicky camera. I can handhold the D3X at lower shutter speeds without camera shake. The much poorer battery performance of the D800 is also a disappointment. The AF capability at f8 is an advantage but I have compared the AF performance of both at this aperture and find they both work fine.
As you can see I like my D3X. But there are times when the higher resolution of the D800 are an advantage.
However with that said I won't get rid of either body. I think it's a tough call, price excluded.