The M1RB allows you to convert the voltage output of Outputs 9-16 of the M1, the outputs of an M1OV, or outputs 9-16 of the M1OVR to relay outputs. The M1OVR provides you with 8 additional voltage outputs and 8 relay outputs. So I don't view it as a question of one over the other, since they serve somewhat different purposes. The M1RB converts the output type, while the M1OVR expands the number of outputs.
Get the M1RB if you aren't using the voltage outputs of the M1 and you need a bunch of relays.
Get the M1OVR if you are already using most/all of the M1 voltage outputs for other purposes and need to expand.
If you are using some of the M1 voltage outputs, but still have at least one that is unused, consider using an ELK 912 or 924 relay. That will allow you to convert just the one voltage output to a relay output for your garage door.
If you need more outputs later, you can always add the M1OVR at that time.