When I just bought my AFS18-200/3.5-5.6VR and before selling off the AFS18-70/3.5-4.5, I did a quick and dirty lens sharpness comparison between the 2 lens. The test was conducted by shooting the characters on my LCD monitor - not exactly an excellent subject but would do especially when I was in a rush. Since there are a lot of interests in the sharpness of these lenses, I'm sharing them here. Please note that there will definitely be performance variation between these lenses and yours. Please also note that this test compares only optical sharpness; other parameters like light drop-off, chromatic aberration, distortions etc. are not considered. AFS17-35/2.8 is added as reference only and not intended to compare with the 2 lenses.
I've rated the sharpness according to my interpretation with the following colors; you're free to make your own assessments.
Light green - excellent
Dark green - good
Light yellow - fair
Dark yellow - average
Red - below average
All images were cropped at exactly the same location - one center and the other lower right corner.
My conclusion from the test: the 18-200VR generally performs better then the 18-70, at least for my copies. The 18-200VR is an excellent lens - there are some corner softness around 24-35mm, but improves remarkably once stop down by 2 stops. My main use for this lens is for shooting landscapes while traveling, and the 18-200VR fits my purpose perfectly.