There is also a M-mount version of the 50f1.5 Voigtlander Nokton coming in a short while. I dont know when they will start shipping - check with Cameraquest on that. Smaller than the LTM Nokton 50f1.5, close focus to 0.7m. I saw the prototypes last year in Japan and it looks - and feels - very good. Performance is supposed to be the same as the LTM Nokton 50f1.5 - which is about as good as you will ever need. I have the C Sonnar 50f1.5, the Asph Summilux 50, the LTM Nokton 50f1.5 and though I tend to use the C Sonnar the most - the Nokton 50f1.5 sees a fair bit of "light" too. They are different lenses, C Sonnar is a black/white dream lens - it does things with the deep blacks that no other lens does (exception being the CV 28f3.5), The LTM Nokton is more of a modern design and as stated, not far off the Summilux 50 Asph - but with less flare - and I can't see any reason why the new M-mount would be inferior in performance.
The difference between a Planar 50f2 and a Nokton 50f1.5 is not that big, but that last f-stop (or fractionally less than a f-stop) comes in handy when you are shooting at ragged edge light - it can be the difference between being able to hand hold or missing a shot.