I saw more difference.
Well I see less chromatic aberration with the 250 than with the 350. And that would be consistent with the laws of physics, because the longer the lens the greater the chromatic aberration,
all things being equal. Since neither lens is an apochromat, it only makes sense that the 350 would have more chromatic aberration, and to my eye it does.
The second version of the 250 is lighter, longer, focusses closer, and has overall improved image quality.
http://collectiblend.com/Library/Leica_Lens_Compendium_Content.phpThe 280 is in a class by itself, with a price tag to match. You don't have to tell me how good it is, I did use it in Germany in 1995 at the Leica Academy.