Cnet posted an interesting story comparing the Galaxy S II Super AMOLED Plus display and the Galaxy S Super AMOLED. They say that viewing them normally, they can't really tell them apart (beside the fact that the Galaxy S II's display is larger - 4.3" vs 4"). On both screens, text was clear and colors in images were extremely rich.
Looking closely, you see a slight difference - because of the PenTile technology of the Super AMOLED (which Samsung no longer uses in the Super AMOLED Plus display). Interestingly, when Engadget reviewed the S II, they said that the Super AMOLED Plus is "nothing short of spectacular" and that it may be the best mobile display on the market.