Prime Lens due to fixed focal, have less glass thus less distortion and stuff. As such image from a prime lens is usually much better then a Zoom. Of course a consumer type of prime may lose to a professional or high grade zoom sometime, but on general a prime lens is much sharper and has a larger aperture. The downside of a prime is of course you have to fit the distance to the lens as such creates some inconvinence. Is nice to use a prime to complement your zoom. Since if you have the 18-200, depends on what you take often, for example if macro get the 105mm f2.8 prime, if portrait the 50mm 1.8, if you want wider theres the 30mm 1.4 or 20mm 1.8. In the end you can use your prime for a more definite purpose and your zoom as a general walk around lens. If you have money though theres always the high grade primes which are extremely sharp...
Do look out for third party lens like the 30 1.4 from sigma. Its quite decent and dont cost alot.