First, you need to do a lot of reading about the two lenses, because they are both good choices.
I suggest these two reviews:
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.
Have looked at 18-200 and 18-300 , need advise