Thanks for the test, Jon. Looks like the most pronounced difference between the two lenses is in the amount of vignetting. The older Summicron 35 versions (preasph. and earlier) also exhibited heavier vignetting. The cost of miniaturization, it seems. The Summicron 35 Asph is not completely vignette-free either (Putts reports -1.8 stops in the corners) but it does have a more even performance than other 35s. It does flare a bit, that's true. Few Leica lenses are flare-proof. On the other hand it does very, very well with colour. Which reminds me -- any shots from the Ultron 35/2 on colour film?
PS. The UK importer of Zeiss/ Voigtlander I usually buy from, sold the first 10 Ultron 35/2 like hotcakes. Whoosh.