Fred,
There do seem to be small (but not insignificant) optical improvements for the A (and, it would be assumed, the FA) over the M, but they are still fairly small differences. And, for some users, they might be small enough that the build quality and focus feel of the M over the A (and especially the FA) might be more significant.
I wouldn't call them improvements - they are just slightly different. Yes, you get more sharpness with the "A" version, but at the expense of the bokeh. The M50/1.4 does have absolutely the best bokeh I've ever seen. It's hard to judge from the web picture, anyway I've tried to support my opinion:
http://www.misenet.sk/Mermaid/
So I have both A and M versions and their 1.7 complements as well. :-)
Bedo.
