The D90 has more features than the D200 like: live view, video capability. If price is a concern I would go with the D200 if its a substantial discount. The only other consideration is that the D200 is a more hefty camera than the D90. Myself I have both and use my D90 for any general photography and my D200 is used for strictly ebay photos
Honestly, the D90 is the superior camera. It has a higher resolution, a better sensor, and more features. The only setbacks are the video mode and the fact that it uses SD memory as opposed to CF. If I'm going to buy a good camera, I'm going to look for a camera, not a video camera. CF is by far the better memory. It is more reliable, faster, and durable. SD on teh other hand SUCKS MISERABLE. They are broken so easily, and write speed is slow. But to each his own. If you want a better camera, for instance a pro camera, I'd go either D300, D300s. They're both great cameras. The D300s being recently released is (naturally) a bit more expensive. But either is great. If you wish to stay prosumer, go with the D90.