For pure gaming, the 9700K is about as good as it gets.
A $800 graphics card would be appropriate.
9900K is similar in gaming performance.
A 9600K could be a very good choice if it lets you buy a stronger graphics card.
What kinds of games do you play?
Games rarely use more than 4-6 threads effectively.
They are both good in different ways, I do have to say if you're just gaming these are pretty overkill CPUs, but if you want to overclock maybe the i7.
Strictly gaming, the 9700k is a solid choice (though, depending on the games and rest of the specs, a tad expensive).
Overall "better" I would chose the 3800x.
For pure gaming, the 9700K is about as good as it gets.
A $800 graphics card would be appropriate.
9900K is similar in gaming performance.
A 9600K could be a very good choice if it lets you buy a stronger graphics card.
What kinds of games do you play?
Games rarely use more than 4-6 threads effectively.
Purely for gaming and nothing else, go for the i7 9700K, especially if you don't plan on upgrading the CPU. With AMD though you may get to upgrade to the next generation CPU and are guaranteed 16 core CPU support with an X570 motherboard.