There are a number of excellent threads on this subject -- it appears to be a subject on the minds of many.
I have both lenses and would not part with either.
The 70-200VR is the best lens in its focal range for sports and wildlife IMO. If you are shooting flying birds, for instance, the speed (F/2.8--F/3.8 with the TC1.4EII) of the 70-200VR is something you will need. Moving subjects require a fairly fast shutter speed (I try to shoot 1/640 or better) and you don't want to have to shoot wide open either. The sharpness and clarity of the 70-200VR will reward your wildlife images. Here are a few very early morning shots with the 70-200VR in fairly low light.
I simply could not have taken the above pics effectively with the 18-200VR because the speed of the subjects required a fast shutter speed, and the light was such that to do that a fast lens was a must.
The 18-200VR is not a fast lens. However, the VR will allow you to take very nice low-light photos of static subjects such as landscapes. Here are a couple shot hand-held with the 18-200VR in very low light. They are tack-sharp despite being shot from a rocking boat (active VR) in low-light sunrise conditions.
The VR was essential to being able to take these shots and it worked great. But remember that it does not help for rapidly-moving subjects any more than a tripod does. As Thom Hogan said, VR used for fast-moving subjects in low light will give you blurred subjects and sharp grass.
The 18-200, however, is the most flexible lens there is. It takes very sharp pics at all focal lengths with nice, contrasty images with bright color. It is SMALL and easy to carry around. If you are looking for a first lens and need to limit yourself to one lens, the 18-200VR now (and the 70-200VR next) might be the way to go. Here are a few ordinary images from the 18-200VR in ordinary light.
I don't claim that there is anything special about any of these images--I'm no pro. They are just examples of types of shots.
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Roger
Huntington Harbour, California
Surf City, USA
'I want to die peacefully, in my sleep, like my Grandfather...'
Not screaming, and in terror, like his passengers...'