I have
M 50mm f1.7, but wish i had the A version. A version might have slightly newer coatings, but that's not a big deal, as all Pentax 50mm lenses are pretty great in terms of optics (and there are some lineages, like the f1.4 lineage and the f1.7 lineage, which go all the way to modern offerings). Its the other features that are important.
A series is when auto-aperture was introduced, and it allows the camera, even modern DSLRs, to control the aperture. This means you get full Av, P and other modes; instead of just M and wide open Av. To me, this is quite a big step, as it can speed up photography quite a bit. The main reason to go for A instead of M is that A often doesn't cost much more, but it has A mode, and it is slightly less old (keep in mind, these lenses are 30, almost 40 years old).
The f1.4 lenses are slightly softer than the f1.7 until f2.4 or so, but by f3 there is practically no difference in sharpness. And f1.4 vs. 1.7
can be noticeable in some situations. So if you want wide open, big bokeh, low light, then go with f1.4. If you want sharpness and a little lower price, go with f1.7. It really comes down to what is available in your local market. If you shoot these lenses at f5.6 or f8, they will be damn sharp. This is what I read from various reviews and experienced myself (I have a couple 50mm lenses, but only the M 50mm is from Pentax)
Also, think about buying some lens hoods. A lens hood can really help these older lenses, especially if you get a lens hood that is tight enough for APSC (can be tighter hood than for FF film). It might even help that 55mm that you have. A good lens hood can help with contrast, colours, and (obviously) flare. You can find really cheap hoods online, even with free shipping from China, if you don't mind waiting a month or two for it to arrive. If you are into old glass, hoods are the most simple accessory that can make a big difference. Especially with digital sensors.
Also, don't use UV filters on digital cameras. Just FYI lol, we're trying to squeeze the most out of these old gems