The answer is very simple, RazerOne49:
if you are willing to carry two lenses around all the time and need the longer reach and the better IQ (from 105mm up), go for the combo.
If you are not
always running around with an additional lens in your bag, you might end up in a photog situation where you will curse yourself if you either have only the 18-70mm or the 70-300mm.
In this case buy the 18-200mm!
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
, My Pictures
, My Photography Blog