I put this question in the non-Japanese question since it's a generic issue, but I DO want to include Japanese watches as well.
By "today" I mean watches made since about 1995. By "mid-20th Century" I mean anything from about the late 1940s thru the 1970s.
However, I'm limiting it to just purely mechanical watches; no electronic, no quartz, no Spring Drive, etc. to make an easier comparison.
The parameters are rather "open", and can include your opinion on movement quality, accuracy standards, accuracy "in practice", durability, quality of casing, ease of maintenance, water-tightness, eye-appeal(decoration as well as overall visual design)...
Now, my thoughts:
Bottom line: I don't think there's a clear cut answer overall. However, I'd say that for the following aspects of watch production:
1. Accuracy standards & care in regulating: better in the "old days". I haven't found comparable warranties (as the Seiko GS VFA and the Zodiac SST offered) among current-day models. Also, to take Seiko as an example, until very recently, it did NOT offer any current model anywhere near as accurate (in terms of it's factory Specs) as the '70s Grand Seiko Special and I think even today it still offers nothing that matches the old GS VFA specs.
However, accuracy (and measurement thereof) is a very complex issue - much more complex than a COSC certification would lead you to believe! I think some of the modern developments [like Daniels escapement] try to answer some of those issues that the "old" high-end watches didn't attempt to address. So, certain modern watches may be catching up to or even with those vintage "super watches".
2. Visual aspects: movement decoration, fit & finish of casing, dial, etc. I'll give this to the better quality modern day watches. However, I also have to say that certain VERY antique (pocket watch) products were just as well made in this respect (although that's not technically part of my poll). You can find beautifully decorated movements (skeletons, highly decorated rotors, striping, guilloche, etc.) in certain current watches that seem to greatly outclass those of 40-60 years ago "on average". Certain luxury brands today offer cloisonne & jeweled dials and cases, sculpted/machined silver dials & cases, and other features that rival the hand-made "luxury" pocket-watches made 200 years ago as "objet d'arts". I think of Ulysse-Nardin as a good example of this - various cloisonne & meteor dialed models and the "Berlin" Rattrapante with the sculpted case-back are good examples of "artistic presentation" in the modern era. J-LC & Seiko (GS w/ remake of the Lion emblem) offer some beautiful casebacks as well these days. I find that was done MUCH less so in the earlier period - though the Seiko & Citizen & Benrus gold emblems (caseback) of the '60s & '70s were impressive in many cases. While some extremely limited edition Pateks & others (like Breguet) from mid-20th Century did have elaborate dials & movements, I think it's less prevalent than today among mid/high to high end models in "regular production" [something found in a company catalog even if very limited in number]. So - I give that point to current watches overall.
3. Overall mechanical durability. Tie or slight edge to modern. The parts selection, fine adjusting, & regulating done by hand years ago in certain high end models probably equates to the precision obtainable today by moderm computer controlled manufacturing processes (with their tighter tolerances obtainable). Thus, less "hand picking" of especially well-made parts from the "average part" in the production run is (theoretically) necessary today. In addition, some hi-end products today ALSO require hand finish-work to improve upon the results of automated precision production of parts [IWC, JLC, etc.]. In addition, better products of advanced materials science are available today than in previous history (including synthetic parts for balance assemblies, etc.) - at least in theory.
Much more could be said & a complete answer could be a PhD Dissertation. I'll stop here & let others have their say Feel free to disagree w/ my opinions as well.
The Patron Saint of Seiko Collectors
and I will just add that modern watches are better built than older ones from a manufacturing aspect.
Movements have been cost reduced for parts and assembly processes.
I think many modern watches could be made to run as well as their older counterparts of adjustment and regulation was done at the factory.
Additionally we can't ignore newer technology parts and materials. Given that so many watches from the early era are still running; we can only wait for 30 years to see if the modern ones stand up. Of course, in our throw away society of today, it's doubtful that mechanical watches will receive the servicing and care that many of them did in the past.
My best guess is that the watches of today and yesterday are roughly on par as far as build quality goes when all factors are taken into account