I have both the Sonnar 50f1.5 and the Nokton 50f1.5 (both in Leica and Nikon mount). The Sonnar has a certain magic in black/white - a "lushness" to it. The focus shift is vastly overblown! If you are shooting in low-light @f1.5 and hand-holding - that affects focus more than a shift in the lens. It is there, but it is minute!
The Nokton 50f1.5 is sharp, truly sharp at f1.5. Better than the 50f1.4 Summilux by a fair margin.
In use, I do use the Sonnar more, mainly due to the size - but for "ragged edge" shooting - I would use the Nokton, or the 50f1.4 Asph Summilux, though the latter is a heavy beast.