I'm currently using the 35/1.4 pre-ASPH and the 35/2.0 Zeiss. I had the 35/2.0 ASPH for years. I dislike the Zeiss as it is too big IMO, especially with the hood. I'd suggest taking a peek at examples from the various options on FLickr, plenty to see there from every M lens. I like to scan through the thumbnails and pick stuff of similar subject matter to what I shoot. As Dick says- final print size also plays into the quality you'll need, though there are some pretty clear differences in the out of focus rendition of the older and newer lenses- and that is taste. Don't rule out the new Summarit if you can deal with f2.5- a friend of mine has one and it delivers very fine images.