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Would love to hear what others have to say about this and the best bang for buck these days when it comes down to raw render time as a work machine.
I am of the opinion that right now it might be best to wait. It is obviously a personal opinion but at the moment if you need more firepower than the 7950x / 13900k packs (which seems to be sitting between a 3970x and a 3960x) the only solution is the 5995wx which as we all know costs a lot. You do get an overkill platform with it, tons of PCI-E lanes, 6 or 7 slots, 2TB of memory support and all that jazz... But if you're purely going after rendering prowess yes, I think you can easily come to a conclusion it is potentially not worth it compared to a 3990x.
3990x however appears to be discontinued, the motherboards are an epic level find seemingly and so it is an option that is hard to recommend imho. Unless you can source the parts and are comfortable with using used & discontinued gear.
So why wait?
Intel is about to release its WS / HEDT parts soon and while it is suspected (by leakers) that these won't really be that much cheaper (and won't be faster than the top of the line TRs) it will still potentially be a step in the right direction because it might have a cascading effect on AMD's lineup. So hopefully at that point AMD will bring out more parts and Intel will at least have something semi-competitive at most price points. At the very least you might have a few more options for anything that might sit between a 5995wx and a 7950x. Plus of course there are the Zen 4 Threadrippers coming out but that is speculated (by leakers) to be towards the end of the year.
So thats just my 5 cents. I could very well be wrong and its the best time to buy now because stuff seemingly keeps getting more and more expensive lol :)