I also have the KM 50/2 and I think it's very good wide open. It's only weakness is a tendency to veiling flare when shooting into the sun, so a lens shade is really a must. The built in shade is useless. I use a Hoya vented 40.5mm hood that I got second hand, very cheap. The quality and colour remind me of the old Nikkor 105/2.5, though this lens is much sharper.
I've never used a summicron, so I can't compare them.
Phil