From my experience with both systems, you might be better off with the 9900K. Reasons:
1) The 9920X, from a stock installation, rarely boosts above 4.2GHz and basically never reaches 4.5GHz. On single-threaded workloads.
2) The X299 motherboard used has fewer options to control the MCE feature that works very well on the i16x series, making it relatively unusable. Therefore you're kind of stuck with the inability to reach full boost on the 9920X. The i16x board with a flip of the switch to enable MCE and a TDP adjustment to allow 125W-160W TDP will work at 5.0GHz on all cores all the time.
3) Corsair have been pretty open that they aren't catering to the enthusiast community with the Corsair One series, so don't rely/expect BIOS updates to help you tinker like for their other product series. Reference: https://forum.corsair.com/v3/showpost.php?p=1000777&postcount=48
Only exception: total RAM usage for your application. You only get 2 slots on the 9900K platform, whereas you get 4 on the X299. I needed 128GB for my workloads, which is not possible with 2 slots. Check your needs on that since it may be the overriding requirement.
Other possibly related factor: my 9900K system has the dreaded extremely annoying pump cavitation noise on the CPU cooling (left) side that many users are complaining about on the forums and in the reviews. My two 9920X systems have no pump noise issues at all, either CPU or GPU (although one of the systems does have some coil whine on the GPU in some workloads). When I rotate the systems around, I can clearly hear a lot of bubbling/gurgling in the 9900K system, indicating the AIO system was not well bled before sealing. I do not hear gurgling in both 9920X systems. Since the cooling systems are identical in both platforms, one might hypothesize that the 9920X series has better quality control...