I see. I personally havent tried both the Nikkon 18-200 and Tamron 18-270, but i kinda prefer to stick to the Nikkor glasses.Also can, the price difference isn't too big.
Just thought the additional zoom from the Tamron is good, and besides, the 18-270 seems to be highly recommended when I did a search on this forum.

If i'm not wrong, at the long end of the Tamron 18-270, at aperture of f/6.3, u'll lose AF capability.