If I want to be honest, Apo-Summicron looks better with one stop stopped down. Even with f/2 it looks maybe not sharper but more contrast a little bit. It is tinny difference there but it is one stop faster. I still like Sonnar *T 2.8/90. It is quite common lens with affordable price and performance close to Apo-Summicron IMHO. I am currious now about results with luxurious Sonnar ZM 2/85 in such comparision. It is still cheaper than Leica and probably has very similar performance :)