Here is a new poll I think to raise. Both lenses are great optically (provided that you get a good copy nowadays for those units AIV (produced in Vietnam)!) and both are actually still in production.
The A50 is half-stop faster but it is just an old MF lens. And on APS-C, its f/1.2 aperture will provide shallower DoF that could be closer to that of a 85/1.4 on 135 Full Frame. The DA*55 is a bit slower but it has AF. It uses SDM, though, which is infamous in accuracy, speed and reliability. The DA*55's focusing throw is long and is no different from a MF lens actually. Furthermore, after the SDM is dead eventually or suddenly, the DA* will be no difference from a MF lens! ;-D
Do note that Pentax/Hoya has never officially suggested that the DA*55/1.4 is anything that is designed for Full Frame (against what I have been suggesting them that they *should* make it clear and then *everyone* will win, since years ago!). In fact, I suspected that the lens hood of the DA* is just incompatible on Full Frame, although as for the lens it seems to be working on FF cameras without any problem.
Both lenses could produce quite good Image Quality on APS-C when just stopped down to f/2 and the bokeh will not be bad (depending on your requirements or simply preference!). The DA* is easier to buy and the A50 maybe available only in US grey market or you need to order it directly from Japanese online shops, like at the Mapcamera. The prices are close enough for both, provided that you can find it!
So, here is the *tough* choice! If you just want to buy a better 50ish now in Pentaxland, what will you choose? I mean genuine Pentax! Not a Sigma or else. Btw, the Carl Zeiss ZK 50 has been discontinued.. Okay, let's go!
Last but not least, whilst all Pentaxians are encouraged to participate in the Poll, which each vote is made anonymously, I encourage even more that you could reply back and post a "Comment" for the rationale and reasons of yours behind for making your choice - There would be some good food for thought and it is always interesting to see for more opinions and viewpoints of what others think! :-)
Related:-
The Very Different Fates of Two ex-AOC Top Optical Engineers
50mm Lens Shootouts
Compatibility of DA Lenses on Full Frame
A50/1.2 is Now Made ("Assembled") In Vietnam
Where are the Pentax DSLRs and Lenses Made?
FA 31 Limited: Assembled in Vietnam Vs Made in Japan - Case Studies (Part 1)
FA Limiteds: Made in Japan Vs Assembled in Vietnam - Continued (Part 2)