And let's face it, sometimes a newly constructed repro part is just a better choice than an NOS part that's been on the shelf for nigh on 70 years...
While I can't speak to the quality of aftermarket vs NOS KF parts (in the odd circumstance where both options exist), I can speak to the aftermarket/repro vs NOS world in a more general sense, and there are almost no cases in my experience where an aftermarket part is superior to a properly stored, good condition NOS part. Obviously seals, gaskets, glass, hoses, other rubber soft parts degrade over long spans of time, but when it comes to hard parts that do not degrade over time with proper storage, the actual original part is almost invariably higher quality. Especially in today's "global economy" i.e. Chinese economy, where the large majority of new parts are made in China these days. That means sub-standard quality control in most cases, sometimes grotesquely so.
That said, there are some areas where the marching on of technology has offered the potential for substantial improvement over original in terms of material quality, or correcting known design flaws with a given part. Brake and clutch friction materials have advanced tremendously in the last several decades, providing a world of different brake pad options, etc. But that comes with a caveat, as just as the potential is there for a noticeable improvement in quality over the original part due to a redesign or using different materials, the potential is also there to go substantially cheaper than stock as well, thus worsening the overall esperience. Cheap parts store brake shoes and pads are a great example of this. Most off the shelf cheap brake shoes now are bonded rather than riveted, and the friction material itself is of poor quality, causing grabbiness and generally poor performance and/or service life.
So, as long as one understands the two opposing sides to every part story - that new does not always mean better, just as NOS does not always mean the best that a part can possibly be despite seven decades of advancement - one can make an informed decision on a part by part basis after asking the right questions about the part in question. One simple thing to keep in mind is, if you are having a hard time figuring out where something was made, odds are it was made in China or else other than the USA. Being made in the USA is a big selling point on most things, so if it simply does not say anywhere on it or its packaging whereit wwe mad, odds are it was China.
Edit: The quality of most repro parts for the big three makes are woefully sub-par, some astonishingly so. High quality repros that are genuinely as good quality as the original are very much the exception, and you definitely pay for them. There is a reason why a lot of NOS parts are big bucks even when inexpensive repros are available.
