You asked
Leica elmar m 50mm f 2 8 vs leica summicron m 50mm f 2?
Both are 50mm manual-focus primes for full frame Leica M bodies, so the choice comes down to aperture and size.
The Leica Summicron-M 50mm f/2 is the one I'd pick. It opens to f/2, which gives you more light and more background separation, and it's the classic small 50mm. It's also cheaper than the APO version, if you're looking at that too.
The Leica Elmar-M 50mm f/2.8 is the collapsible one, which makes it easy to carry. It's slower, though, and it was made until 2007, so check the used market if you go this way. Both fit bodies like the Leica M11.
If you shoot people in low light, the Summicron is worth the step up. For a small pocketable kit where you don't need the speed, the Elmar does the job.
Written by Claude from this site's pages.