I sold my M3 on this Monday. True Leica comparing to M6 (IMO), but I'm not 50mm user. I went for less expensive, cheaply build, but absolutely new Bessa R2M for 35/50 framelines and TTL. But I also have M4-2 and working, serviced professionally Zorki.
IMO, M2 might be much more practical comparing M3 and more Leica comparing to M4-2.
Just remember what suggestion to be ready for $$$ CLA, repair bill if buying M of the ebay is as real as it was never been before. Even M6 TTL is old camera now. And some electronic parts are not available anymore for this flavour of M6. M6 classic seems to be not in the jeopardy, yet.
Be careful what you are wishing for. Barnacks and M are in different categories for CLA, service cost. And after using of both, Barnack and M, I have feeling what M isn't as tough and as long lasting as Barnacks are. IMO and if camera is in use, not on display
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