Can anyone explain the performance difference between the two? I have found several good clean copies and am trying to decide which I should consider.
Another option would be to get the Voigtlander Ultron and the 50 Nokton 1.2 or 50 Lanthar APO
I was able to compare the Elmarit ASPH. with its immediate predecessor at time of acquisition and I can say in all honesty that from the get-go, I preferrred the rendering of the older lens; that said, the ASPH. is an amazing lens for its size - sharper (in the center), contrastier, super-compact. However, the "pre-ASPH.", while quite a bit bigger, delivers balanced and detailed shots that are very appealing, smoother to my eye, more natural, and with more even performance across the frame. It also has a distinct signature I really like and it handles wonderfully (at least without its humongous hood), that's why I still have it.
However, here's the clincher: I prefer my Voigtländer Ultron 28mm f/2 (type II) over both Leica Elmarits; it's a fantastic performer that comes at half the price new I paid for my Elmarit pre-ASPH. used ... and it's considerably smaller while being a full stop faster. The Voigtländer shows more vignetting, but sharpness and rendering are both at least equal or even more desirable than the Elmarits IMO. Indeed, after acquiring the Ultron, my search for high-performance 28mm lenses basically came to an end.
Depending on your objecitves and needs, I think you can't go wrong with the two f/2 lenses you mention: The Voigtländer APO Lanthar is my best 50mm for M mount (and I only own one 50mm lens that actually beats it, the Nikon Z 50mm f/1.2 S, a lens four times its weight and thre times its price). And, though maybe because I don't own that many 28mm lenses, the Voigtländer Ultron 28mm f/2 (II) is my best 28mm lens, full stop. It's also amazingly small. Both lenses are very well made, too - yes, Leica lenses are still smoother in most aspects, but some may actually prefer the somewhat firmer action of the Voigtländers.
I also own the 50mm f/1.2 Nokton - a great lens, but more of an artistic tool than a top performer. However, it's a very, very good, acceptably compact f/1.2 lens (remember, I own the Nikon Z!). You basically get three lenses in one: a super-fast 50mm for low-light shooting and super-shallow depth of field, an f/2 to f/4 performance that equals the famed Leica Summicron-M 50mm f/2 (a lens I also own) for portraiture and general people photography, though the Nokton is obviously quite a bit bigger, and at f/5.6 and f/8, it's sharp across the frame to allow for all kinds of documentary photography (and street, including zone focusing). I'd say that it beats the Summicron beyond f/5.6, and it's no mean performer at f/1.2 to boot.
However, my pick would be: the APO Lanthar and the Ultron. Both are "pinnacle" lenses in my book ...
Regarding the 28mm lenses you mention, this is my understanding. The ASPH renders modern, is very sharp and is a wonderfully small size. As regards the non ASPH, the most recent one (iv) renders similar to the ASPH but is bigger. Versions i to iii of the non ASPH lens are Walter Mandler designed lenses, also bigger and to my eye render gentler, a bit more pastel. The problem is I can't really help you as all of these versions are great and there's no way I could recommend one of the versions over the other. The one I desire most which combines pretty much most of these traits, is half the size between the larger and smaller Elmarits plus a faster aperture in one package is version one or two of the Summicron ASPH 28mm. But that can cost between three to five times the amount of an Elmarit.
Gentlemen, thanks for the prompt response. I imagine I will be on the phone with the vendors this week. I do like the compactness of the ASPH version but what attracts me to the Pre-ASPH is the Wetzlar CLA. So from a performance aspect, it should be good as new.
Maybe a slight remark as o preferences of lenses: as the resolution of sensors change then the lens preferences change too.
Many comparisons were originally made with the M8/M9, but now with the M11 everything might be different.
I now appreciate the high res lenses much more than I did in the past.
I switched from 50 Summicron V3 to V5, regretted it, but now V5 shines.
I have an Orion-15. Set at F:10 you would expect a crazy depth. It does not. It is sharp in a small plane only; my ZM 28mm will have more depth of field. So sensors matter as much as lens schema's.