If the nature of your work requires the wider aperture - either for low-light situations or for subject isolation via shallow DOF - the 'Lux versions of the lenses are worth the extra cost. (The lens naming scheme assigns Summilux to the f/1.4s, Summicron to the f/2.0s, and Elmarit to the 2.8s.) Some of the newer 'Luxes also have aspheric elements, which adds to sharpness, designated as "ASPH". The 75mm 'Lux is somewhat of a special case, as it's an older design with a distinctive "signature". All of the f/1.4s perform well wide open, but do increase in sharpness even more when stopped down a bit.Originally Posted by Wouter from Brazil
If size and weight are more important to you, the Summicrons or Elmarits may be better choices, however. Weight specs are available on the Leica site.