If you didn't have the motherboard already, I too would suggest AMD Ryzen. I just got a new Ryzen 3600. The Ryzen 5 3600 is just 20-30 dollars more and trades blows with the 8700k.
In fact though, benchmarks on this site show the 3600 trading blows with the 8700k.
https://respawnfirst.com/amd-ryzen-5-3600-vs-i7-8700k-benchmarks/
In USD, I paid right around $180 for the 3600. But just saw it on sale for $150 and the 3600x for $180. To put it in perspective, the i7 8700k would cost over $300 if I were to attempt to purchase. Whereas I can have a motherboard and Ryzen for less than that.
If you look here, this gives an idea of the 9700k vs the 3600 and other chips.
https://www.gamersnexus.net/hwreviews/3489-amd-ryzen-5-3600-cpu-review-benchmarks-vs-intel
What I would say is this. For gaming today, i7 8700k in my opinion is done. Good performer, but to expensive vs what is available.
The 9700k is faster and I'll give you that for sure. But the issue the 9700k has is AMD's Ryzen 7 3700x. The Ryzen has hyperthreading whereas the 9700k does not.
For gaming, the 9700k is faster. Full stop. The issue the 9700k will have might be staying power. As more games come out that use more cores and more threads, I think the 9700k is going to age faster than the 3700x will.
But in either event, if you already have the motherboard, the i7 9700k is faster, it just depends really on how much you are spending on it.