First, you need to do a lot of reading about the two lenses, because they are both good choices.

I suggest these two reviews:

http://www.photozone.de/nikon--nikkor-aps-c-lens-tests/242-nikkor-af-s-18-200mm-f35-56-g-if-ed-vr-ii-dx-review--test-report

http://www.kenrockwell.com/nikon/18-300mm.htm

Remember to use either lens at it greatest zoom, you should have you camera sitting on a sturdy tripod.
Both lenses are heavy and could be a burden if you lug it around all day.

Ask yourself if you really need all that zoom.

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