What's your budget? And are you looking for the sonnar signature, or do you simply want a faster 50?
The Nokton 50/1.5 is a wonderful lens, modern look, sharp and crisp (incl. wide open), but not overly contrasty. A sonnar, though, it is definitely not.
There are a number of threads on the new Zeiss C-Sonnar 50, which seems to be a stellar lens. Check out the Gallery and flickr pages of mfogiel, who uses one a lot. Also the M-Mount group on flickr. I'd love to get one but it's well out of my price range.
The Canon 50/1.5 is a sonnar-clone, and a wonderful lens, both for portraits and, stopped down, general use. Looks and balances great on an M2 as well!
🙂
The Nikkor 50/1.4 LTM is a sonnar-design, too, IIRC, as is the Jupiter 3.
In the end, I sold my Nokton 50/1.5, but have kept the Canon pictured above.