The technology of the D3x is about two years older than that of the D800, If you go with the D3x you could be missing the enhanced high ISO performance of the D800 and a wider dynamic range. The other not frequently extolled virtue of the much higher pixel rating of the D800 is that often you may find there is more than a single image in your frame; i.e. sections of the frame can be as valuable as the whole of it. Further more, if you get the grip for the D800 you may not miss the feel of a pro body. I am doing that on a D700 and have tried the same with a D800 as well. In the end, the image quality difference is more that the paper specs tell.