I have a D3x, and a 5DII.
I regularly use the cheap 50mm lenses on both cameras. Theses 50mm lenses are addictive.
The D3x weighs a ton, is built like a hammer, focuses fast and accurately within a cm or so tolerance, and produces beautiful luminous images in any light.
The 5DII is fairly light, is solid enough for the real world, focuses decently but not really well, and seems to run out of light all too often.
In practice, the 5DII seems to have better DR to handle external architecture and interior architecture. The 17m TS/E lens is delightful.
In practice, the D3x does much better on portrait situations with a medium tele, due to the fast off-center focus points, and skin tone is something I prefer. I have run off full length portrait posters from the D3x Jpegs.
More subjective drivel.
- Direct-printed postcards from the Nikon look better.
- A quick test I did with a fast C telephoto (200/2) on the 5DII showed inaccurate focus acqusition. A set of tests I did with the N 200/2 on he D3x showed inferior sparkle/look, compared to an old C 200/1.8 model which I have used extensively for fashion and portrait. The old C 85/1.2 is superb but slow; the N 85 1.4 is a sharp and fast-focusing lens which always gets a decent shot.
- The 17mm TS/E on the 5DII is superb. If you want to use this lens, it's worth acquiring a camera for it.
- The 5DII does video. One day you may need that.
My feeling is if you want to photograph things, and in particular interiors or exteriors with highlights and shadows, you want the 5DII, if you want to photograph people especially working in low light or people who move, you want to use the pro Nikon. Both cameras can do either quite well, but each does one thing superbly. I also believe that the old 5D may be superior to the 5DII for skin tone. As for the EOS-1 series, they're nice but not perfect. The D3x is still the closest thing to a perfect all round camera that can be found out there. And yes, I still use a 50mm on each camera most of the time.
Edmund