Short answer-Buy the Aspheric if you can afford such a thing (and stand to part with the $$'s).
There weren't any bad 35 Summicrons. The Aspheric is as good as it gets. I have the old 8 element Summicron, and am very happy with that. Not as zingy, but a great lens (to me, the best b&w lens). Unfortunately these are overpriced collectibles now. The 7 element pre-aspheric was the one called "the King of Bokeh"; it is a fine lens, but not quite the cult lens it once was, so down in price a bit.
Personally, the 6 element version is my least favorite. It is a great lens, but more emphasis on contrast, and less on edge sharpness (compared to its 8 element predecessor).
I have no personal experience with it, but from everything I hear, the Zeiss is a great lens and a great buy. Maybe the best choice in the current market if you don't want to spring for the aspheric Summicron.
Last edited by Mark Crabtree; 11-02-2011 at 10:14 PM. Click to view previous post history.