From the USPS web site NewPage1
it becomes a little clearer about what the difference is. The USPS and BOATUS are *only* offering an MMSI. Vessels that go outside of the US will also require a Ship's Station License, which for many years was $75 for ten years. Then it jumped to about $225 for ten years, now it is $160. (Still a rip-off, the FCC is supposed to charge what it costs to administer.)
So when you get a free MMSI, you don't get a Ship's Station License. And if you go outside the US and buy the required SSL, that comes with an MMSI of it's own, which must
be used on all equipment and must
replace the free one if you had one. Making the one a waste of money, if you're going outside of the US.
So the "difference" in the MMSI is no difference at all, just that one is just an MMSI, and the other is an MMSI associated with a required SSL. Either should
work perfectly well, no matter where you are, even if there's a technical violation about which one should be on the boat.