First ..all the 50 s are superb lenses . I can t think of any summicrons ,summiluxes or noctiluxes that aren t great lenses . The new APO summicron is clearly capable of the highest IQ of any Leica lens ....yet ...I have never found myself feeling that the 50 summilux asph was in any way deficient . So assuming you could buy either...how would you decided?
The 50 s do however differ in their character ...is the bokeh smooth and how does the transition to OOF areas roll off . Do the aberrations contribute to the aesthetic or are they distracting . How strong is the contrast and what is the balance between resolution and micro contrast . How is color rendered and does the lens impart a signature "tint" Sean Reid described this a few years ago in test as being similar to an artist selecting from his paint brushes . This is what causes the endless discussion of clinical verse flawed renderings .
The 50/2 APO is not yet in general circulation so you have to rely on the early tests and sample images . It seems a given that new lens will set the standard for the technical view of IQ ....MTF at given apertures , resolution edge to edge , micro contrast and freedom from aberrations . This will translate into image rendering that is exceeding sharp and approaching perfection . This of course will be most visible on the MONO because of its ability to render the finest detail of any M body .
What is not discussed much and needs to viewed ..is how this translates into color photography . You can see in the Noctilux 0.95 a brilliance and color purity , a fineness in the micro contrast that separates the tones that exceeds the 50 1.4asph . The best samples I have found are from ByBrett ( linked on the Leica User Forum) . Here the performance of the new 50 APO seems to render color similar to a Noctilux .
My feeling is that the elements of IQ that are defined by the MTF charts ,review of edge sharpness and apparent image contrast exceed my requirements and abilities (focus,camera motion etc) . But those that define the character of the lens ...bokeh,color fidelity ,aberrations etc can be seen in every capture .
Based on what I have seen so far and ignoring cost factors ....I would prefer the newer 50APO but I am not selling my 50 1.4asph anytime soon.