The 35 Cron Asph is very sharp even wide open and has great bokeh, but it is also a bit to the contrasty side relative to the other choices. The 35 Lux Asph is a tad softer wide open, but sharpens up to the very nearly Cron Asph performance by f2, and at f2.8 and beyond they run head-to-head on resolution. The Lux has great bokeh and more for my tastes, is a bit softer contrast than the Cron. The IV is an older design and has a very smooth look to it. It is sharp in the center wide open, but fades to a slight softness towards the edges. Bokeh is butter at all apertures and stopped down to f4, it's hard to tell from the Lux Asph in sharpness or contrast, so you kind of get a two-fer use lens. And it is tiny.
Choosing a 35 is also a big problem for me.
I had the 35 Lux ASPH which I loved on film, but even after tuning in Solms was only on at 1.4, then 5.6 and smaller. At 2.0 to 5.6 it focus shifted. When I use the 35 on the M8 it is normally at fairly close distances and the rearward focus shift really bothered me.
I switched to a 35 Cron ASPH, which Leica adjusted for me so well that it really never causes any issue for the focus point at any aperture or any distance. I really like the size of this lens, its sharpness, and bokeh. However, It can be too contasty and transitions to the OOF bokeh are too abrupt for my taste.
The 35 Cron IV is a little less contrasty compared to the 35 Cron ASPH, but has the Nocti type bokeh in some instances and flares more than the ASPH. So it is out, as I just can't deal with the Nocti bokeh it gives sometimes.
The new Summarit is not in contention for me, although it is a good lens I truly believe that the Cron ASPH gives more fine detail and should be better in the corners with a future full frame camera (lets hope one comes along eventually). The MTF curves also substantiate this.
The Zeiss Biogon is out as it is even more contrasty than the Cron ASPH, and uses a different thread for the filters than a 43 mm Leica does. That means you have to use B+W filters which give a different filtering / in camera correction than the Leica filters do.
I have not tried the CV offerings in 35, but just seeing the size of the 35 1.2 knocks it out of consideration for me.
At this point, the 35 Cron ASPH stays in the bag but I normally shoot around it with the 28 Cron ASPH or the 50 Lux ASPH.
My wish is that Leica would reformulate the 35 Lux ASPH to give the same rendering & contrast it does now but without the focus shift issue. If they do, I'm buying it.
Ray