My vote goes for 'Cron ASPH, at least this is the one I acquired just a week ago from Tony Rose after torning between the choices (Cron vs Biogon) for several weeks.
I had no doubt about optical quality of both, read numerous reviews of both (including Reid reviews for subscibers), but there are several reasons that made me spend nearly twice as much for the Cron as new Biogon costs:
1. From numerous examples by Cron ASPH vs Biogon and Cron v4, I noticed the former one produces more shallow DOF at given apertures and I liked that - appears as some kind of 3D appearance. Both Biogon and Cron v4 tend to go a bit more smoothly into OOF, which sometimes may also appeal, but I prefer that obvious separation that new'Cron ASPH offers. Might be pure aestetic issue.
2. According to several technical reviews, 'Cron ASPH might be slightly fallter in field (i.e. slightly more unifors sharpness into the corners), but yet that might be more academic that practical.
3. Cron ASPH is physically smaller (without hood). Size does matter (to the opposite direction though 😉) for me, since I tend to go for street shooting with two Ms, one with either 50mm or 90mm, the other will be either 50mm or 35mm.
4. I really like the square hood of 'Cron ASPH - just so sleek and great looking one, not to mentioned more optimized then regular round hoods.
What I'm not sure I'll so happy with - focusing tab. I used to regular camera/lens handing, with my left hand supporting the camera from the bottom with fingers controlling focusing ring and I feel really comfortable this way with my 50mm Summicron. Now, with that tab on 35mm Cron ASPH, it appears I'll have to alter my regular handing to get acustomized to tab (single finger) focusing....
That is somethign that would not probably bother me would I choose Biogon...