I just got caught up reading the thread.
To me the most important aspect of your rig, at least underwater is the
lenses you use. Get the best you can and as Eric suggested and I concur, get in the water and start
shooting!. Use cheap optiques with either one of these cameras and the best results will not be there. The proof in quality lies large printing (24 x 30 or larger). as much as all the other features mentioned here.
I want images that once in my library can be, at any time enlarged greatly while retaining all the elements I saw when I initially create the image.
I am confident I will earn the difference in price between the two if I continue make a living with my camera. If you are not shooting all the time then the D200 is a better choice.
As I mentioned before, lighting,mood and an eye for capturing the moment are more important than the difference between these two cameras. The question is can you make them dance for you!
1- Get a D200 with better optiques and use the change for a spare body
or
2- If you have it (Mulla, $$) spend it on the D2x. with the best lenses off course.
It is true, as Kasey suggested that I seldom compare slrs openly.
I had my D100 for over two years, then the Kodaks for about 2 as well, so it is time to move on again... Since I am in the industry it is also important to show and work with the latest, as long as it is the best for me as a professional. I love my Kodaks and they have served me well.
One thing that has remained constant over the last 25 years are my lenses. I continue to use my 16mm manual lens even with the latest digital. It is a supperb piece of glass. Nikon allows this
As the mount has never changed.
Since I use my camera more on land than I do underwater I want what feels right as well.
My vote goes for the D2x for 99 reasons.
If I was not making a living at this ....then the D200 would be my choice.
Now go and get wet!