Ramon V (California) , Sep 12, 2009; 02:17 a.m.
if you are happy with the 18-70mm on the D90, why not just get the 70-300mm VR to complement it? or even the cheaper but very good performer 55-200mm VR.
the only two advantages that i see of the 18-105mm VR over the 18-70mm is oviously the VR which is useful on the long end when and where needed, and the additional 35mm stretch. the latter will give you the less chance of changing lenses for reach.
on the other side of the story, the 18-70mm beats the 18-105mm (in its 18-70mm range) in speed and i can say (this is just me) image quality. the 18-105mm has less distortion than the 18-70mm at 18mm.
you can check KEH for reasonable prices of the 18-105mm.