I have both, and both are wonderful but for different reasons.
Before you ask, both work on Film as well, just make sure you have a film body that controls the aperture with the DA50.

Now to the nitty gritty.

FA50 F1.4 - Heavier of course, but better build. Not much bigger than the DA50. Lens does extend with focusing. Has the focal distance in a window.
IQ? Softish wide open. I would have said soft until yesterday when I used it wide open on my K-01, it certainly not soft. Lacking contrast possibly, but easily corrected in CameraRAW. CA is mad wide open in certain situations. sometimes not. Depends on the light. Stopped down to F2.5 and beyond, this lens is sharp and then some.

DA50 F1.8 - Cheap but great. Plastic mount, yet very light. Internal focal movement meaning the lens does not extend. More contrasty wide open and better control of CA wide open. However the FA knocks spots off it from F2.5 with its contract and sharpness. Then again it is not total. There is something like the old SMC-M F1.7 about it though, a certain magic in the images. (I have the F1.7 as well)

It all comes down to what you want with the nifty. The FA has the extra 1/3 stop and in the right light gives you a magic in DOF. It also provides far superior IQ when stopped down.
On the other hand the DA50 is more contrasty wide open and better control of CA up to F2.5, but then again that is when the FA is superior. Then again it is less than half the price now, and far lighter. albeit cheaper build.

At the end of the day, If you can afford the FA, go for it. If you can't, get the DA. It is still a fantastic lens and produces wonderful images. It really is not an inferior lens to the FA, instead, just as good but has different but wonderful results.

It is an absolute cracker!
Build, tilt screen, supermacro, the LCD, image quality, handling,
So much so, my mother bought the last one at Jessops in Manchester, and she loves it too (a rare thing)
If there is a down side, it is the lack of hotshoe. But apart from that it is excellent and knocks my Panasonic LX-5 into a copped hat.

It is a common problem, the cog breaking. My MZ-50 did it as you described. So I bought an MZ-7 to replace it. So far so good. But I am wary of the cog going.
I was given an SFX and it has done the same thing.
Fingers very crossed, my MZ-5n, which I have had since new, will last a bit longer.
I have heard about someone who will replace the offending cog with a metal one, but at a cost.
Whether it is worth it, is up to you.
For me, I won't repair the MZ-50, it is not worth it, as it only have a plastic lens mount. MZ-7? Possibly it was cheap but clean and a great camera. The MZ-5n. Certainly. It was my last newly bought Film SLR and cost a fortune at the time (£300 in 2000) and it is a great camera. So I don't care about cost, if it gives me a reliable film body.

To be perfectly honest. I would suggest that rather than trawling through the classifieds or forums with the hope of obtaining what you want cheaply. It would be an awful lot better to actually invest in a brand new HD-DA 1.4x converter.
As your namesake would say, it is "quite remarkable"
I know they are not cheap but you do get the following:-

1. Full weather sealing
2. The latest HD coating
3. The SDM/DC contacts
4. The screw drive for none SDM/DC lenses
5. It is full frame capable.
6. It is stunning with most lenses. Well I say most, there are a small number that you can't use it for due to the rear element moving so far back in use it would hit the converter glass.

I know it is not cheap, but seriously it is the best K-mount ever.

I hope this helps.

In one word - Yes!

The beast is mighty. Sharp and contrasty from wide open. It is not cheap but worth every penny.

It is leaps ahead over the 55-300. Mind you I would suggest keep the 55-300 as well for its size and flexibility as stopped down it is still an excellent lens. But the DA*300. WOW. Just invest in one and you will realise why everyone who has one rave about it.

I did'nt get your name, nor you mine, but you did say you are a member here, so hello.

I hope that the 50mm Macro and 20mm F4 will give you lots of great images.
Look forward to seeing some of the results.

I can confirm that yes. I have one and works perfectly on film without any sort of vignetting.

It is a tough decision really. It all comes down to what you want to pay and how much you need the lens. Why? Well lets give you some examples

The SMC-M 50mm F1.7 has a current street value of around £20-40. Probably better built, or at least feel better built. But then again no A mode so green button is needed.

The SMC-F 50mm F1.7 are fetching the most money. The first AF version and current ballpark is around the £120 mark for a good example.

The SMC-FA version, rather rare, but with the FA50 1.4 still available new, may have compressed values a tad. Last one I saw sold for about £80.

The SMC-DA 50mm F1.8. The brand new digital configured lens, that will work on FF, and if you have certain film bodies, will work brilliantly on them. Then again the bayonet is only plastic, but it is very light so that is not an issue. New price? £115.

Personally, if it is top drawer then £60 is more or less right, if not, probably around the £40 mark is a better price.
However it is your decision, and only you can decide the value of what is an excellent lens.

It all comes down to Image Quality versus Flexibility.
I did the Duxford airshow last year and had my DA*300 and Sigma 170-500mm.
Most of the time I used the DA*300 due to where I was positioned (outside the airfield) and 80% of the shots were taken with it. A lot were cropped due to the lack of reach ironically, I would have preferred to have my 1.4 converter with me, but I only bought it in March this year. The other ones were where the aircraft went over our heads and the Sigma or indeed the 55-300 would have been the better option.

Personally, and it all depends where you are at Airshows, go for the 300 first and accept the fact that some images will be too close (or creative as the case may be) and relish the IQ in the first place. Then consider a Sigma for flexibility for future. Then again, find a spot that you don't have to have the Sigma in the first place and enjoy knowing that the 300mm will give you what you need.

Hi and welcome. I am another Cheshire Tog, yes I still consider Stockport part of Cheshire