I’ve got a Matrixbrute. The best use case I can see for owning both a Matrixbrute and a Minibrute 2, if money is no issue, is connecting the two, using the Minibrute 2 as an extra voice for four-voice paraphony. To explain, the Matrixbrute has a paraphonic mode that allows you to play each of the 3 oscillators as a separate voice, for 3 voice paraphony. This is different from polyphony - say you play 3 notes at once, and a chord rings out. Then you add another note; one of those previous notes gets cut off abruptly and the new note begins to play. In practice it’s different, and trickier to get the hang of, than true 3-voice polyphony would be, but you could do some very cool things with it.
Anyway, I believe (but don’t take my word for it) that you could easily patch the Minibrute 2 into the Matrixbrute so that the Minibrute’s oscillator becomes an extra voice, for 4-voice paraphony. That would make a big difference in playing, I think - you could have a 3-note angelic string pad on the Matrixbrute, then press a lower key, and the Minibrute 2 voice could kick in and be your suboscillator low bass sound to complement that, all on the same keyboard. Pretty cool. People are doing similar things when they chain together multiple Mother-32s for added voices.
If you’ve only got money/space/inclination for one of these, it depends on what you want to do. The Matrixbrute is a dream for sound designers and deep programmers, and the mod matrix / sequencer grid is powerful, flexible, and super accessible. It might be overkill, though, and to be honest, there’s not a whole lot going on with the sound design that I can’t do with my Prophet-12, although the Matrixbrute has a very interesting character all its own that lets me dial in, say, gorgeous 1970s kosmische arpeggiations very easily. The Matrixbrute has less CV connectivity than a Minibrute 2, too; if you’ve got Eurorack the Minibrute 2 might be your best friend - it looks like it’s taking on the Mother-32 more than anything else, with that generous patch bay.
Worth pointing out that the Matrixbrute is huge in physical size. It kind of instantly becomes the center of your studio, and demands space and your attention. I have a small apartment and it’s hard to justify the sheer visual space the thing takes up, and out of practical necessity I’m moving heavily towards desktop-based hardware going forward rather than big mothership synths. Something to consider. That Minibrute 2S actually looks super cool in terms of form factor if you don’t need dedicated keys.
Not sure how helpful I’ve been - I think between the Matrixbrute, Minibrute 2, and Minibrute 2S, Arturia has a stable of synths that are all cool and worthwhile in their own ways. I’d vote Matrixbrute as solid classic for the ages, though. Check out Marc Doty’s two Matrixbrute courses on Ask Video if you want an idea of how deep you can go with it.