Originally Posted by Phillip P. Dimor
There are multiple differences between MC and PE glass. If given a reasonable price go for PE. It is newer has half stop detentes and upgraded lens design. I believe the MC glass is the oldest. There is EII glass and PE glass. I may be missing some iterations. I am just mentioning what is currently rolling around in my camera bag. Besides the half stop settings some I've read there is no noticeable difference in prints between PE glass and EII glass in some focal lengths. There is definitely a difference between a 150mm PE and a 150mm 3.5 MC.
ETRS lenses have gotten so cheap recently I would go for the newest models. The reason is they are less likely to need servicing. Servicing an ETRS lens now is not a great idea because it costs more to service than just buying a new one.