I only know the Modular, so I can't tell. It is possible that Arturia used a slightly different algorithm for VCO and VCF in the MiniMg so that there exists a reason to purchase it as well.
Theoretically both should be the same, in software as in hardware. But when you read discussions about the sound of different Mg systems (hardware), different MiniMg serial numbers, and the new Voyager, there seem to be - according to the users - huge differences. There are musicians that own several different Mg modular systems, at least one MiniMg, and a Voyager. It is their secret why they had to buy them all, and probably it is also for collection.
Personally I never had the wish to have the Arturia MiniMg, I think I can create any sound from it with the Modular. But I may be wrong . . .