The interwebs is full of assumptions that Leica > Zeiss > Voigtlander
Concerning optics, as a practical matter (hand held, B&W film), really only in price. When people chuck out a lot of $$$ into something, they feel the need to justify -- I mean, come on, they cost so much more they must be "better." I use to be one of them too, having the best, but now it just seems kind of silly. I like Leica optics for B&W film, but they're not so much better or so different to drop a couple grand more on a lens. Twenty-five years ago we didn't have a choice. Today we are truly living in the golden age of rangefinder optics.
A little off topic, but after using many different cameras over 25 years, I've come to the conclusion that what was best about Leica, historically, was not there optics, but the viewfinders. The Leica M3 trounced what came before. Similarly the Leicaflex SL remains the best viewfinder I've ever used in an SLR. Once I had a variety of f/.4 and f/2 50mm lenses in rangefinder mounts from the late 1950's from Leitz, Nikkor, and Zeiss. From my limited examples (1Z, 2L, 3-4N), Zeiss > Nikkor > Leicas. Anyway, anecdotal, and I'm really not up to speed on modern Leica optics, although I had the 50mm Summilux Asph when it came out and that was truly outstanding, albeit large.
Although cost for Leica stuff is a bit of a red herring too, because it retains value. I sold all of my m mount lenses 10 years for more then I paid for them new. This assumes Leica keeps raising prices; a fair assumption.
My whole issue with the ZM lenses is that they tend to be either too damn big or too damn slow, with the 50/1.5 being a notable exception. Never owned any of the ZM's though. Nice optics from what I see on Flickr.
