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 ...
M.