I used to own the FA 50/1.4, but in my opinion it didn't seem that exceptional. And while I have not had the "M" version to compare, I
did have and compare the M 50/1.7 vs the F 50/1.7, and that M lens held its own very well against its autofocus sibling; both lenses are very sharp even wide open. However, the manual focus ring on the F lens was very small poorly damped, and had a very imprecise feel, compared to any M lens (IMO). Strangely enough, when pixel-peeping the comparison between the two, they were identically equal in sharpness, but only when using
autofocus on the F lens- again, the quality of the manual focus on the F kept it from doing its best.
So, based on my experience, and with your plan to still mostly use manual focus even with the new lens, I would say that it wouldn't be worth the expense. But if you have the money to burn right now, you might consider a slightly different lens than the FA 50/1.4. Especially for your party scenario, maybe something wider? You might be able to find a good used FA 35/2 for that price, (or consider even the DA 35/2.4). And despite the its poor manual-focus attributes, if you want to stick with a 50, the F 50/1.7 can often be found for about $200 (savings!
), and it IS sharp and does have great autofocus, even in lower light. Something to consider. Good luck with your decision!