i7-8700K vs i7-9700K

With a kind of modern i7 somewhere around 4.5 and beyond I wouldnt upgrade either.
Time will tell if an 4.2GHz 12core will outperform a 4.5-5GHz 6 or 8 core.
With Vulkan and possible mGPU in mind it was wise to not bet on a 20-lane horse but consider something that can truely fire 2 x GPU and a bunch of NVME’s, all with their native speed w/o compromising other hardware in use.
This means either a WS class chipset from Intel or AMD.
I have now had 3 x Desktop style boards after a decade of WS parts and I wanna go back more sooner than later. Limitations everywhere you look.

I just built an i7 9700k based system ASUS ROG Strix Z390-F with 32GB 3200mhz RAM and GTX1080 video card and 2, 1TB SSDs… This cpu is amazing, low latency 96Khz no issue … cubase 10 pro works great on this configuration… all my projects work without an issue… lots of headroom…

The Intel Core i7-9700K is also rated with a base clock of 3.6GHz and can boost one its processor cores all the way to 4.9GHz. Comparatively, the Intel Core i7-8700K starts at a tick higher 3.7GHz base frequency, but can’t go nearly as fast with its maximum 4.7GHz boost clock.

I would go for i7-8700k for it’s hyper-threading feat, though some might not need it, it still a big plus for me having it and I’ve seen some tutorial that i7-8700k can also go to 5.0ghz if overclocked properly.