There are a number of variations in the Summicron. The original Collapsible used Thorium glass, then moved to something less radioactive. The Collapsible, Type I Rigid, and Dual-Range Summicron where 7 elements in 6 groups. Lower contrast, high resolution. The Type 2 Rigid went to 6 elements in 5 groups, a bit higher contrast than the older ones. Finally, the Summicrons went to 6 elements in 4 groups- Same as the 1930s Summar. It is higher contrast, probably sharper. But many users prefer the "look" of the older glass.

Type 2 Rigid Summicron, wide-open on the M2:

http://www.l-camera-forum.com/leica-...ing-lunch.html

Price-wise, the Collapsible Summicron and Type 2 Summicron can be had in the $300~$400 range for a nice one. The Type 1 Rigid goes for a bit more, and the recent ones are in the $1000 and above range.