I had a 35 Summilux pre-ASPH. It started as a loaner from Leica. Had planned on a new Summicron (we're talking mid 70s here) switching from a googled version I had for my M3, and the Leica rep suggested I try the Summilux. When it came time to turn it back in the rep asked if I'd like it for a great price and so it became my main lens for about 20 years.
When the ASPH version came out I traded it for one thinking I just wanted a lens that 'did it' without thinking about stopping down for sharpness. It wasn't long and I was thinking I'd made a mistake, I liked the compact size of the pre-ASPH and it had a look I liked or was just used to maybe. The pre-ASPH 35 Summilux I've described as an 'art-lens' and 'two for one'. Opened up it has a beautiful feel and look and gives a special image quality, 'dreamy or 'atmospheric' could describe it, stopped down to f5.6-8 it can be as sharp as you probably need and contrast is greatly improved. And yes, it can flare, and you can't see this with any knowledge of what it is doing with a rangefinder. If you use one its just part of its characteristic you except.
I did buy another pre-ASPH at one point and used it. One thing to keep in mind too is there are sample variations. Some are just 'better' than others.
As for my ASPH... I've had a couple, both in the heavier brass mount titanium and chrome. Its a great lens and you don't need to think about sacrificing image sharpness and contrast in the f1.4-f2.8 range like you do with the pre-ASPH. Never had a problem with 'crazy' flare and I'd sometimes be using it without the hood.
I'm finally with a 35 Summilux I really like, the 1st version 'aspherical'. At 300grms its only about 50grms heavier than the pre-ASPH, not as compact of course, but acceptable, and (like the ASPH) its sharp and good contrast at any aperture with a nice look, smooth bokeh, and handles nice with its tab (smaller and more comfortable than the edged new style tab) AND focus ring.
So I would say going through all 3 versions of the 35 Summilux... if you want a unique look, compact, fun lens that can challenge to 'think like the lens' you go for a pre-ASPH. If you don't mind a bit more size (its still not a big lens), but just want a lens what will optically do it all the ASPH is a fantastic lens. Price and availability aside, the 'aspherical' is my favorite 35 Summilux, optically about the same as the ASPH, but handles better (for me), and a bit better built, no problems with 'crazy' flare like the pre-ASPH, but still has a bit of that old school glow.