The 10900KF will do 4.8 GHz all-core at $472, the 3900X will do it at around 4-4.1 GHz[0]. AMD has a slight IPC advantage in productivity, Intel has a slight IPC advantage in gaming[1].
The market for the 10900K/KF is basically someone who wants the fastest gaming rig, but then as a secondary priority wants more cores for productivity. Kind of an overlap with the 3900X, sure, but the 10900K/KF will still outperform it in gaming, where the 3900X tilts a little more towards productivity.
There are of course exceptions, CS:GO loves Zen2's cache and Zen2's latency makes DAWs run like trash, so your specific task may vary.
I'd personally point at the 10700F as being the winner from the specs, 4.6 GHz all-core 8C16T with slightly higher IPC in gaming than AMD, for under $300. That's competitive with the 3700X: little higher cost, and more heat, but more gaming performance too. The 10900 series and 10600 series are a little too expensive for what they offer, the lower chips are too slow to really have an advantage.
But really it's not a good time to buy these anyway. Rocket Lake and Zen3 will probably launch within the next 6 months, if you can stretch another 6 months there will be discounts on Zen2 and more options for higher performance if you want to pay.
[0] https://www.pcgamesn.com/amd/ryzen-9-3900x-overclock
[1] https://www.techspot.com/article/1876-4ghz-ryzen-3rd-gen-vs-...