My current system is 12 years old with a Core2 Q6800 processor, so I have no doubt that either will be a substantial upgrade.
 
My question is, for MAINLY Audio/Video processing and some VST instrument use, would 8 cores be better, or 6 cores w/ 12 threads? Anything I read about these processors seem to be very gaming-centric, and they say very little about practical applications.

Thank you kindly

There is not a lot of information on the streets yet on the 9700K, and much of it seems to be a wash in most cases. The only things that truly peg cores is running encryption software and video rendering, and the only data I have seen so far the 8700K won by a large margin on encryption (due to the thread advantage). I did not see anything on video rendering, but would assume it is similar. In reality, you would be hard pressed to find either CPU limiting for DAW usage, but I would personally take the highest thread count (8700K). You would make a jump of 8x your current benchmark with an 8700K even without overclocking.
 
That said, bear in mind that the motherboard and supporting hardware also play a large part in system performance, so there is more than just the CPU to consider with a new build.

There are lots of threads here (and other DAW forums) regarding recent successful DAW builds -- do a search.  If you're in the U.S. I'd recommend giving Jim Roseberry a call and let him advise you or (better) build you a system.
 
If you insist on doing it all yourself, keep in mind there are always risks aiming for the "latest and greatest":  For the really high prices, you may not get the power (or worse, compatibility) you expect; where as, last year's popular components will be much cheaper and still ridiculously fast.

Here is a review of the 9900K that I found.
https://www.tweaktown.com/reviews/8768/intel-core-i9-9900k-9th-gen-coffee-lake-review/index.html
 

Neither...  
8086k
Six cores (12 processing threads) at 5GHz.

Thanks for the input! I hadn't even considered the 8086, it's now on the list, but every review I saw said the extra performance wasn't worth the additional cost. Is it? It's hard to get good computer info anymore for non-gamers!  I do think the i9 is overkill for my purposes, it would be nice, but beyond my needs.
 
I am considering an ASUS Z390 motherboard also, but I'd never allow someone else the joy of building this PC...I've never done it before, but I certainly have enough savvy to do it, and I've been using this tricked out Dell XPS 410 from 2006 for the past 12 years, I really wanted a current system to use. I just don't really understand hyperthreading vs. actual cores and which one is better at what. I do a fair amount of video and audio conversion and rendering also, and this PC has a hard time with all of that, but my i5 at work seems to handle it a lot better.  

I bought the i7 8700 (not K because of the diff in heat and price . 8700k runs 95watts and the  8700 65watts .But this is interesting 
BTW, Some say heat isn't much diiferent between the two, others say it is.
https://cpu.userbenchmark...i7-8086K/3937vsm516988
 
i7 8086K also is 95 watts. BTW. But, I am happy with the 8700.
 
But it does stink that intel keeps canginging the sockets-i9's need 2066 or something. i7's 8th gen are 1151...All about $$$$$

Gargamel314
I do think the i9 is overkill for my purposes, it would be nice, but beyond my needs.
 

This assessment is an accurate one. My 8700K is overclocked to 29% and it has only ever pegged when being benchmarked, and the reality is that much of that extra umph is wasted. Almost all of its life has been below 50% (I actually cannot recall it ever breaking 50% yet other than on benchmarks and video rendering, TBH).
 
The other things to consider are these:

1. RAM - 8GB is the bare minimum, 32GB has been overkill for me. 16GB is a nice medium (again, I am usually running less than 50%) and you can add more later if needed (if you get 2 8GB sticks initially, then you would still be able to use them if you get 2 more to save on upgrade cost).
2. Motherboard - I have become a fan of ASUS, but even for these CPUs, there are many to choose from. If you research, much of the differences are splitting hairs (and some folks have done nice reviews of differences). The easiest way to sort them is by the features you want with the board (ASUS has a nice search feature here, start on the left edge filters if you want to look at them).
3. GPU - you will definitely want a dedicatated GPU if working with video to remove some of the strain off the CPU. Again, you can get into hairsplitting here, so performance/price point is a good way to decide on all three of these items (and even the CPU).
4. If not hard-pressed for a new machine NOW, CyberPowerPC is a good resource to monitor and sign up for emails from. They run specials on components, so you can get upgrades cheaply. When I got this machine I Goolged for a coupon that actually took at checkout (10% off). They also have a massive warehouse, so I escaped the GPU price-spike which was lucky, but I am not sure what the market looks like now.

Lastly, this is rarely mentioned but important, is workflow. For those of us who grew up on limited-resource machines, we have learned the art of judicious resource management. That translates well into this day-in-age when developers assume users have excess resources available and get sloppy with developing. I think that many issues people see is the "fear of commitment," where they will let numerous VSTs run each and every time the transport does "just because." Something simple like noise removal I will always render out; it is silly to assume I might want that noise back. Rendering/baking just adds another wav file to the audio folder, and with good naming conventions (projects, tracks, and clips) it is easy to find things even years into the future. I just wanted to mention this since even with a high-end machine, you can get it to choke by being a "sloppy user."

I recommend "buy overkill" to my clients. Probably not in the i9 category for my "senior" clients but if the pocketbook can stand it I would go with the most.  Current applications may not need it but who knows what the future brings.
 

Another vote for Asus. I have been using Asus motherboards for my builds forever.
In my post above #7 I reference a website called userbenchmarks. The benchmark app is free and the users upload their results. so the info is for real, not hyped.
You can compare cpu, gpu, hard drives, ssd, etc, etc. Great site.