If you dont take a lot of telephoto, I would suggest you get the Tamron 17-50mm glass. Very sharp and worth for the price you pay.
Imagine your 18-200mm glass is set at 200mm, it is quite challenging to take shots w/o camera shakes. Most of the time, you either use a fast shutter speed or mount the camera on a tripod to counter the problem. But then again, using a fast shutter speed and mounting on a tripod may not be possible in some situations.. Personally I'm speaking from experience because I use a 18-200mm glass as well and I seldom/try-not-to shoot telephoto...