I have the C Sonnar 50/1.5 and the Nokton 50/1.5 LTM and enjoy them both. I find that they both have their own character.
I find that I use the Nokton more for B&W and Portraits and the Sonnar more with color in street and landscapes. Although I did photograph some city scapes from a 22nd story building and the detail that the Nokton captured was outstanding.
Bottom line I enjoy using both. The Nokton is a lot of bang for the buck. The same can be said for the Sonnar when comparing the price to the Summilux.
Good luck with your decision.