I asked the same question a few weeks ago, but with a view to moving from a 2.8 Summaron to a Summicron.
This was Tom Abrahamsson's reply...
"
Unless you need the speed, the Summaron is one of the best of Leicas 35's. The close up performance is better than the Summicron and from about f4 to f11 they are equal. The Summicron, when it came out was a compromise. Leica needed a fast 35 as both Canon and Nikon were beating them at their game. You gain a stop, but you loose a bit of edge, particularly in the close range.
Later Summicron's improved, but not until you get up to 35/2 Asph. do you get the same close range sharpness.
For the money today, the Biogon 35f2 is the best deal.Much more pleasing tonality than the 35/2 Asph and about the same resolution and less harsh contrast.
An early to mid 90's Summilux is another good choice. It will get you 2 stops, but it is not as "straight" as either the 35/2 or the 2.8. It has a curved field, particularly when you are using it wide-open or close to that. Sharpness is otherwise similar to either the f2 or 2.8 pre-Asph 35's.
I have the whole gamut of these lenses and one of my M2's always has the 35/2,8 on it! Usually with Tri X in it and it is rare that I cant get a shot I want with it. OK, nothing is perfect, the infernal infinity lock can drive you nuts, but I usually dismantle these on any old 35 or 50."
Regards
Ernst
