i5-10600K or i7-9700K??

That's not a question you can take in isolation.

It depends on the rest of your system, local pricing, and your uses. For instance if you can get a respectable Z490 motherboard for not too much than respectable Z390 boards, the 10600k seems to make a lot more sense.

But if the rest of your system is quite mid-range, spending oodles of cash on either plus the expensive mobos makes little to no sense.

Or depending on your specific uses (not just gaming, but what kinds of games, what resolution, refresh rate etc) something else altogether may make more sense.

I'd suggest following the advice here and doing a full request for comments on a build / upgrade.
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