I've found it hard to make the Summicron flare- I've been shooting this a lot all summer in the woods often inadvertently getting the sun in the frame between leaves. With a filter on trouble is much more likely. Shooting in Boston a few weeks ago with an 021 or an ND filter I had I some veiling and some flare when shooting into the late day sun. I wouldn't blame this on the lens but rather on the filter, especially since it doesn't happen without the filter...
In prints I'm just now working on at 16x20 with the Summicron I can see tiny leaf stalks and crisp detail in birch bark, even though it is quite small in size. Quite stunning performance. Yes there is some vignetting wide open, but in practical use it isn't an issue.
The Summicron also shares better 3-D rendering with the Elmarit over the Biogon, handles reflections beautifully, and is smaller in size.