They are both excellent lenses. I prefer the 35mm for its extraordinary sharpness. Its better than the 50mm when shot wide open (note: I have the 50mm 1.4, not the 1.8, but I've read they are about the same at 1.8). The auto-focus is faster as well. The only weakness on the 35mm is its tendency to produce purple fringes. The good news is that unlike my D40, the D90 corrects this automatically.
My recommendation would be to take some time shooting your 18-200 at each of the focal lengths and decide for yourself which you prefer. The focal length is the biggest difference between the two lenses.
