Tough choice between the two super zooms. One other consideration is whether you will be taking any shore excursions or tours, and if so, what will be your primary photographic interests? If it is architecture, you might find the wide end on either 18-x00 zoom a bit limiting. Understanding that you would prefer to take but one lens, nonetheless, the 10-24mm AF-S Nikkor 3.4-4.5G ED could be a good edition, allowing you much more flexibility. On a DX sensor it's the difference between approximately 15mm and 27mm on a full-frame 35mm sensor. It is relatively small and light weight. The 18-200 attached to a D300s, and the 10-24mm, fit nicely in a Crumpler 5 Mil. Home, with room for a spare battery, filters, etc.
Sorry, I cannot give you any input about what to ask for your present lens.