Just so happens I did a test two weeks ago. I had 3 Leitz 135mm lenses - Hektor, Elmar M, and Tele Elmar (E39) and wanted to keep only one, but which one? I used an M8.2 on a tripod and mostly shot a newspaper taped to the wall and various objects none worthy of posting and I deleted the files anyway. The results: The Hektor to put it kindly is a portrait lens with lots of glow wide open. Stopping down a few stops sharpens up the middle but the edges and corners remain soft.
The Elmar really surprised me. I thought the Tele-Elmar was going to blow it to the weeds but the results were really close, so close that I sold the TE. The TE was late production from 1994 with perfect glass. The Elmar the one I decided to keep is M mount from the 60's. I made 6 shots of the newspaper with the camera on the tripod at max aperture with each lens, and for each shot I always refocused. Out of the 6 shots I chose the best one. I thought that should eliminate focusing error the best I could. I then compared the chosen shots from both lens. Aside from the corners the details from the newspaper were the same. The TE was perfect and the Elmar corners were slightly soft. I only tested them at max aperture so in the real world when stopped down a stop or used handheld it would not matter.
There are of course other characteristics such as bokeh, contrast, color, etc., I did not test but given I only use this focal length a few times a year at best and that I could and did get $750 for it I did what was right for me.
