Just thinking long term, the extra threads of the Ry3700X and its capability to OC itself with decent cooling, makes it a preferable choice over the 9700K in my opinion.
If you want to do other things than gaming in the PC, then the Ry3700X is a great choice. Make sure you get, at least, 2933Mhz RAM (already have seen this is the minimum speed you want to use with Ry3K) and maybe 16GB of it.
Also, you don't need a crazy expensive motherboard to use it! You can buy a B450 with good VRMs and still have the same "high end" experience with the CPU as the X470/X570 chipsets. You will only forgo PCIe 4.0. If you're not that price conscious, then a X570 motherboard would be ideal.
Cheers!