Whether Leitz/Leica or Zeiss is not the question or issue. Any of the 35mm Summicrons will provide sharp results as well as the newer Zeiss. The issue is really age of the lens and whether there is any mold growing, de-cementing of elements and other age related issues. I had an 8 element Summicron RF Leitz lens with googles for my M3. Compared it to my Leica 35mm Summilux ASPH at 5X with K64 and could not see the difference.
Purchase the newest version of whatever lens you decide on and purchase by condition for the money rather than any other attribute.-Dick