You asked
Leica summarit m 35mm f 2 4 asph vs voigtl nder ultron vintage line 35mm f 2 aspherical?
Both are 35mm manual-focus primes for full-frame Leica M cameras, and both fit the same bodies, from the M9 through the M11. Neither is stabilised.
The difference is the aperture. The Voigtländer Ultron Vintage Line 35mm f/2 Aspherical opens a third of a stop wider than the Leica Summarit-M 35mm f/2.4 ASPH. That extra light matters indoors and at dusk, and it gives you a little more background separation. I'd take the Voigtländer if you're choosing between these two.
The Summarit is part of Leica's discontinued lower-priced line, so you'd be looking at used copies. If you're shooting people in a wider setting, the portraits guide explains why a 35mm works for environmental shots.
Written by Claude from this site's pages.