I have all the Leica lenses mentioned. 3.5 is nice and small and there is a small round clip on shade . Diaphragm is hard to set inless you have small fingers. 2.8 are lowish contrast at 2.8, work well at 4. Summitars, I have two, are very sharp at 5.6, soft wide open. Summicron is a bit better than than the Summitar, but not stellar. ANY version of the Summicron is way better than the collapsible. I compare only top condition lenses.
My favorite is the recently discontinued 2.8 Elmar M It is my go to 50 for daylight 50 mm.
Old lenses in top condition are hard to find and any savings are eaten by repair costs and frustration.
Summars are uncoated from the factory, hence low contrast. Mine takes charming pics when I want the effect.