I shot some tests with an 18/3.5 Ais; It had distinctly stronger color saturation than the 18/4 Ai I had been using. My used sample was defective in focus, so I didn't buy it. My only issue with a good copy would be the clip-on lens shade, in both availability and efficacy. The pictures are good.
My eventual choice(for about US$300 more) was a used Zeiss ZF.2 18/35, which has a nice deep petal-style shade.