Hi Alberto
I have both lenses but I only shoot film, so I can just give you a quick summary of the differences as I see them (of course you may get better advice from someone who's used them on the M9):
- Both are a joy to focus via their tabs.
- Both of my copies are very well built although ASPH feels more solid with extra weight.
- IV is really compact & light. Smaller and lighter even than the C Biogon 35mm.
- IV doesn't have the sharpness of ASPH at wider apertures, particularly wide open.
- IV vignettes quite a lot wide open (unlike some people, I don't like vignetting).
- Bokeh on both is excellent (some people don't like ASPH bokeh but I do).
- ASPH has more micro contrast but it's not overly contrasty.
In summary, these are two of the nicest lenses you can find and I think your choice may depend on what kind of image you like -- technically perfect (ASPH) or more traditional (IV).
Lawrence