it will be used for gaming mostly
i heard the 9600k was incredible and its really good value for money
The i5 9600K is a great CPU, but we already have games were the lack of HT already show the limitations of this chip.
To put it simple, buying a 6 cores / 6 threads cpu at the end of 2020, is because you know for a fact that the games you play don't need and wont need more than those 6 cores, and you are not planing to play any of the newer games that really push over 6 cores. And of course the fact that the only worth it upgrade will be a Core i9 9900K, if you can find one at a reasonable price later on (Im guessing they will cost equal or more than gold in the next years hehehe).
Edit:
Just to make things clear, we already have plenty of games that push over 6 cores, and you can see this if you look at the 1% low FPS, which as a result of the limited amount of cores/threads giving worst results than those CPU with 6 cores / 12 threads or higher.
For me 1% lows is whats important, if what somehow indicate game smoothness.