9900 is a solid choice especially since I've seen the 9900kf come down to $420 recently, so if you already have a PCIe graphics card that could be an affordable option.So we still leaning towards the i9900k here?
Personally, I've become sold on the new Ryzen so I'm going to give the 3700x a chance and the great thing is if I want to upgrade to a 3950x in the future I can. With the 9900k, you're at the end of the line and future upgrades will be a whole new platform. Honestly though I think every time I've decided to an upgrade I just built from scratch so in practice the idea of future proofing is more of a "peace of mind" thing than a reality.
The other reason I like the 3700x is the lower power draw. I've always tried to invest in CPU's that don't get as hot as the sun, which is why my last system was a 4770k. I've never had a single issue with it, and that was with a stock Cooler. This time, however I'm going with a Noctua cooler just to be able to push the CPU a little more.