I own the new Zeiss 50mm and have used the 35mm a bit. The Planar is as good a lens as any 50mm I've ever used. I don't own a current Leica Summicron--mine is a Dual Range from the late 1950s--but I'm familiar with the various versions. The Planar is at least the equal of any of 'em. Consistent performance across the frame at all apertures along with smooth out-of-focus rendering and typical Zeiss neutral color (to my eyes anyway) & snappy contrast are the lens' hallmarks.
The new Biogon has the same general character as the Planar. It's a sharper lens than my friend's 4th gen. Summicron, at least in the corners, from f/2–5.6. Can't say how it compares to the Leica 35mm ASPH 'Cron. Sean Reid's tests indicate the Biogon is sharper in the corners at wide apertures than the ASPH Summilux.
IMO these comments verge on being nit-picky. Any of the lenses I've mentioned will give you great results. I'd say skip the test charts and go for the lens that delivers the overall look you like best. For b&w I'll be sticking with my DR 'Cron and my Contax-mount Zeiss RF lenses. For color the Planar and 35mm Biogon rock, and I'm very interested in how the 21 & 28mm Biogons perform too.
-Dave-