Multi or single threaded, 3700X is roughly on par with the 9900K and the 3900X is just slightly better in single threaded and way ahead in multithreaded. Just read any of the many reviews and you can see it for yourself. the new AMD processors are also better in performance/watt and performance/dollar.
Just read the reviews.
It seems like everyone here thinks the more RAM the better. That’s not always true. It depends on the type of projects you work on. If your models are architecture, interiors, or generally big, complex and / or have lots of parts then more RAM is better. But if you are a product designer and work on single objects / one part at a time, then you will probably never really need more that 16GB. I had 32GB in my previous build but never ever even came close to utilizing only half of it. Now I have 16GB.
Of course if you use insane mesh settings, you can use up crazy amounts of RAM, but then I would suggest to find out more about using mesh settings in a more reasonable way.
You write your average file size is 2-10GB “Gigabyte”?! Well, if that is indeed the case, you really should go for MORE RAM.
Also make sure you buy fast RAM (clocks and timings). it benefits intels as well as amds processors (though amds profit a bit more). 3600Mhz is the sweetspot for the new ryzen 3000 CPUs.
Here is a review that focuses on workstation performance. should help you more than most other reviews that focuse more on gaming performance:
I hope that helps you make a decision.