I’m basically in the same boat as you and looking at exactly the same boards. Here is some info I gathered over the past few weeks:
Both boards feature a true 8 phase VRM layout for the CPU. The Designare uses [email protected] power stages. The Taichi uses [email protected] power stages. At stock speeds both are fine, the Designare might run a little more efficient. For overclocking the Taichi is better, since it has more headroom. Both boards use the same power stages for SOC VRM. Gigabyte has a two phase layout, whereas ASRock uses a 3 phase. Since SOC doesn’t pull much current even when overclocked, it shouldn’t really matter, both are fine.
Source: LGA TR4 X399 VRM Liste
The Designare doesn’t have an external clock generator (unlike its cousin, the Aorus Gaming 7) so no BCLK shenanigans here. The Taichi has one. Otherwise, both boards seem to be competent overclockers in the CPU as well as the RAM department. I have no idea if and how Gigabyte implements PBO for TR2. ASRock seems to have added the option in the latest BIOS, however it seems to be a little buggy still.
Source: Manufacturer websites, various threads on overclock.net
ASRock has had and still has a few buggy settings in its’ BIOS, but frequently releases updates and beta builds with new features. Gigabyte’s BIOS seems stable in comparisson, but is very slow at releasing updates.
Source: Manufacturer support page, Asrock x399 Taichi/Fatality overclocking and unofficial support
The Taichi has excellent IOMMU support, see here: ASRock X399 Taichi / Fatal1ty summary Thread. I haven’t found a list for the Designare but I imagine it to be similarly good. Just remember that on Ryzen platform in general, everything that is connected to the chipset is in the same IOMMU group. That includes the x1 slot on the Taichi and the x16(x4) slot on the Designare, unfortunately.
The Designare is beatiful. It has 3 more fan headers and a USB 3.1 Type C front panel connector.
The Taichi looks meh in comparison. However, it has two USB 3.0 front panel connectors instead of one and it has extra power for the PCIe slots in the form of a 6-pin connector below the SATA ports. Also there is a U.2 connector on the edge of the board (shares bandwidth with M.2_1, the one to the right of the x1 connector). Lastly, there is a convenient BIOS flashback button in the IO area.
In my opinion, the Taichi is the better board because it has better power delivery to the CPU, SOC and PCIe and therefore more overclocking headroom and/or longterm durability. BIOS flashback is more easily accessible (it’s there on the Designare, but no button in the IO area). I like the connector layout of the Taichi more, even though the sound connectors in the rear are in a weird spot… The BIOS also has more features, knobs to turn and things to fiddle with, albeit being a bit buggy at times. Also I like the layout better than Gigabytes’.
All in all, the boards are very simmilar. Pick the one you like. For me, I like to overclock and fiddle with stuff, so I’m leaning to the Taichi myself (the Fatal1ty Pro Gaming actually, it’s the same board only with 10G ethernet and a COM port).