i7 9700K or i9 9900k for my next build?

Personally when I build systems I don't like a compromise that could lead to regret at a future date. I went with the 9900k. If you're considering it already, then you likely can afford it, so why not? it WILL perform better than the 9700k in literally anything that takes more than 8 threads and it will be relevant for much longer as a result.

What's your budget?
Basic rule is, buy as much CPU as you can afford.

If you're going to spend $400 + another $200 on a decent motherboard, you might as well drop the extra $100 and get the 9900K.

It's faster for productivity stuff for sure. It won't make much difference in games for a while, and in fact gaming is actually running faster with HT disabled at the moment (since it's affecting max OCs by 100-200 MHz)... but if nothing else it is a better shot at a high-binned 9700K, with an extra 4 MB of cache.

Just gaming? 9700K

9700k will be fast at games for 4 years and 9900k will be fast for 5 years, that's the kind of difference you're looking at. there is no difference today and maybe for 3 more years

I have almost the same build as you're planning, and I decided to go for a 9900k instead of a 9700k because my earlier chip (3570) was a locked i5. That plus the lack of HT really screwed me over years later.

Even if HT doesn't give you much extra performance in practice (30% of a core at best) it's still really helpful for anything that just needs a couple % to run and go back to sleep, like VoIP.