The Gigabyte B550 board is better. First reason is it has a strong 5-phase Vcore VRM that's more than capable of handling a 5800X3d; even a 5950X would be comfortable.
Second reason is the B550 chipset that allows you to mount both a PCIe gen 4 NVME AND a PCIe gen 3 NVME. Of less value is it also allows PCIe gen 4 operation with the latest GPU generations.
It should also have a complete suite of hardware monitoring for on-board devices, including VRM temperature and DIMM voltage. Asus is pretty bad about cutting out those important sensor lines for low- and mid-range boards in their lineups.
Other than that you'd have to compare other features like USB ports, both generation and number as well as LAN max speeds if that's important.
And lastly: the B550M board is mATX so it will fit an mATX case whereas the B450-PLUS II is ATX and won't. Either will fit an ATX case.