What you have now is probably most important towards deciding what to get next.
If you've got a basic kit lens like 17-55 or something, than I'd probably consider the 18-200 to be the best pickup as it gives you one-lens versatility for all kinds of photography. ; Then, work on finding a cheap low light prime down the road. ; If on the other hand you already have, say, a two lens package that covers the range (eg: 17-80, 70-200), then I'd say to add the low light prime first since that's an area you don't have yet, then work on maybe replacing the two lenses for one when you make your next purchase.
For me, I bought my camera and an 18-250 at the same time, because I knew I wanted a one-lens solution as a walkaround and travel lens. ; I later added the primes for low light to fill in the holes.