Honestly they're about the same. The 3700X will be faster in some applications and the 9900k will be faster in others. I personally would go with the 3700x. If you get an X570 board you get PCIE 4.0. Also you have the ability to upgrade the CPU since AMD has already stated that the next Ryzen processors being released later this year will be AM4. And you don't have all the vulnerability issues that they are finding on the Intel processors. Not to mention the 3700x is cheaper.
The only way I would consider a 9900k over any Ryzen processor would be if you have a 2080ti and game at 1080p high refresh rate. Otherwise it's Ryzen all day long.
It's hard to justify the i9-9900K for the price unless you're specifically gaming. I went with an i9-9900KF because it was on sale and I didn't want to deal with possible BIOS issues with Ryzen.
The upgrade path feather in AMD's cap was very meaningful a couple years ago when AM4 was promised to be supported for a few years. With no guarantee for new AM4 processors beyond this year, I'm not sure how much that matters with a Zen 2 system.
With the 3800X being much closer in price to the 3700X than it was on release, it's easier to justify the price premium.