I've used both.
As the others have mentioned, it largely depends on what your needs are.
The one thing that the D200 have over the D90 is its built quality. Personally, I prefer the handling of it more than the D90.
Other than that, the D90 has better ISO performance, latest GUI and also a larger LCD. It's also a lighter body, which some might prefer.
If you're just starting out, the D90 might be a better choice for you. Unless, you want the better build and the ability to meter with the older Nikon lenses. Not sure if you can get a D200 brand new nowadays, but the price would probably be close to a D90 body.