Both look like viable choices for serious overclocking. Their 2-slot (single DIMM per channel) RAM layout is claimed to reach DDR4-4800 compared to 4400 on the 4-slot boards. That's the real advantage of these two. In the past, the Extreme, Apex, Gene and usually Formula shared ASUS best BIOS. The difference between them is size. Most of that is in PCIe slots. I'll have to check the spec pages when published to see whether the smaller Gene has compromised on VRM phases or capacity.
There are other viable choices, too. With BIOS upgrade the 9900K's will run on Maximus X boards, but he Z390 is supposed to overclock the 9900K family better than the Z370. I have run the M10A at DDR4-4400, but not higher and the timing sweet spot is about 4133. For my competitive overclocking, the OC Panel is an important accessory. There is no connection for that handy tool on Maximus XI boards and no indication of a replacement for the OC Panel's functions. That's a strong driver for me to stay with the M10A.
The EVGA Z390 Dark has a unique approach to VRM size and cooling and placement of DRAM's -- CPU socket is turned 90 degrees. They rate potential DRAM speed to 4600MHz. Some functional improvements make up what EVGA was lacking for convenient manual overclocking. With the absence of the OC Panel on M11's, the z390 Dark is looking like a closer competitor for my needs.