Back on topic. For most things, most people will do, most of the time, an 8 core 3700X is fast/good enough.
But, the price increase to get a full 12 cores (3900X), has become so relatively small (12 cores, use to cost a huge fortune), that it could well be worth it. Even if it only occasionally helps you, speed wise.
In the UK, it is little more than a £100, to go from an 8 core (3700X), to the 3900X. I would imagine, that equates to around $100 to $135 more, in the US.
I was impatient and picked up the 3900X today for $400 locally. I've got 15 days to return it if something nicer poops up.
You could well have made the best/right choice there. The real-life speed improvement, may well only be a small amount. It is best to NOT buy the latest AMD processor, just/immediately after its release, as problems (not necessarily big ones), can and do crop up.
You may have to wait a while, for suitable motherboards, which have the correct bios updates (if necessary), to support the new cpus, when they arrive.
The new trend of AMD charging a fair bit more, for the XT versions, rather than the X versions (e.g. 3900XT vs 3900X), makes me suspicious that when the new processors first arrive. There will be a price penalty, which may not be worth it, and later (not that much later), the price could settle, to the same price level it is now.
Also, there can be shortages of new processors, when they have just been newly released, which again could have messed you up.
In theory, if/when better AMD cpus come out, they will work (until your socket type eventually gets outdated, perhaps in 12 months time), in your system. So, you could upgrade to the latest/best cpu, and sell your old one.
In reality, the 3900X is so powerful, I don't think you really can go wrong with it. 12 cores, is going to be good for a long time. Most people, are still on 2 or 4 cores. Even 8 cores, is still fairly/somewhat rare. Possibly a modest improvement in single thread performance, is a possibility, with the new cpus, when they arrive.
As time goes on, more and more software, will be able to use all 12 cores, usefully. Unfortunately, there are limits to how much software can use so many cores, because of various factors (Amdahl's law
https://en.wikipedia.org/wiki/Amdahl%27s_law ), but also how difficult/expensive it is to write software which does a good/best job on all 12 cores.
Because relatively few people, have lots of cores, and all these difficulties with creating software that is that powerful, that it can use lots of cores, well. You are not likely to see that much software, which does use it well.
But games, are a promising source of software that does, because they tend to use game engines. So if the game engines can/do use all the cores, usefully, then that could work out well.
Anyway, games depend a lot on the graphics card, which is a completely different ball game, and not part of this thread topic, so we can leave discussing that for another day.