With the D300's excellent high-ISO performance and the generally well-lit conditions in amusement parks, the 18-200 would offer you more versatility I would think.
I'm not sure where in the world you are, but in the western hemisphere, I would think that there is quite a bit of ambient light from the sky and reflected in the water until rather late in the evening.
Ultimately it depends on the kind of shots you would like to take of course, so my comments are just generalizations.
The VR on the 18-200 also allow you to get slightly longer exposures without the tripod, thus enabling you to get light-streaks from rides even on handheld shots!
I hope you'll share some shots with us - amusement parks always make for some exciting shots.