It comes down to two things: plate (for stab orientation) and footprint (for whether or not it'll fit into the case)
The plate varies case by case, so it's hard to know for sure without testing. While stab orientation may not matter in the Mira, it might matter in an LZ CLS or an Eqaulz 80.
The B.87 standard is smaller than A.87, and the USB port is mounted differently on the Z-axis. This, again, comes down to a case-by-case basis. Some cases will have the USB port in a place that allows for easier access of a B.87 port, and some cases may not have the space to allow for an A.87 port.
The problem comes in with the TX87, which uses a slightly modified A.87 standard. This makes it closer to the size of a B.87 PCB, while still maintaining some aspects of an A.87 (stab orientation, USB port placement).
So, by my understanding, for any case that is confirmed to work with TX87, B.87 is the PCB to buy from this GB due to the slightly modified USB port placement to better suit A.87 cases. However, it mostly comes down to the case you're using. In some cases, the stab wires can only be used in one direction (TGR A.17 comes to mind)
This is my understanding of the situation.