Alright -- here's two pictures taken with the same camera, same settings, on a tripod, only changing the lens and then refocusing. One picture was with the Nikkor AF-S 18-70 f/3.5-4.5 G ED; the other with Nikkor AF-S 18-55 f/3.5-5.6 G.
The main differences spec-wise between the two lenses are: the 18-55 has a larger minimum aperture (the 5.6) compared to the 18-70. The bigger difference is indicated by the "ED" on the 18-70. It stands for "Extra-low Dispersion", which is Nikon's higher quality glass formula.
Settings were 200 ISO, aperture priority, f/8. Zoomed to 50 mm. There's a slight difference in the angle, as I probably moved the camera slightly when changing lenses. Also, the 18-55 shot was at 1/40; the other at 1/30. Camera decided that, possibly because of the camera movement picking a slightly different exposure.
I focused on the center of the yellow flower in the middle of the frame. See any differences between the two?