Puts did a very long comparison a while back.
http://www.imx.nl/photo/leica/lenses/lenses/lenses/page86.html
Scroll about halfway down.
Personally I own the v4 and a Summilux ASPH 35.
My friend owned the Cron ASPH 35 and I had a chance to shoot with it.
The Cron ASPH is brutally sharp at any stop. Seriously, you could cut yourself on these negs. I would say it's about a stop or two ahead of the v4. But from f4 on you'll have a hard time telling the two apart. The ASPH draws flat as a pancake. Distortion is close to zero. Because the ASPH is much higher corrected the transition between in focus and out is more abrupt than with the v4.
I think the biggest problem is that the v4 still is a really, really good lens.
Basically you have to choose between one lens that is exceptional (v4) and another that is superb (ASPH).
To me the biggest difference is the way these two lenses draw. The v4 is a classic design by Dr Mandler. It delivers strong contrast, but glows a little more than the ASPH. This is really beautiful in direct light with b/w film. Tonality is very creamy and rich. Bokeh is smooth with a gradual transition in to out of focus.
The ASPH has a totally modern look. High contrast, very, very sharp, very little glow around highlights. Very well defined tonality.
Think of it as the difference between Kodak TMAX-2 400 (ASPH) and Tri-X (v4).
Oddly enough the signature of the Summilux-ASPH falls right between the v4 and Cron ASPH. In terms of performance it acts like a Cron ASPH opened up a stop.
PS: Do not use the small square plastic hood on the v4. It locks in to place to stop it from rotating, but this can act as a wrench and loosen the front element. Stick to the round vented 12585 (?), that can spin feely.
