I'm pretty limited personally in my knowledge of both Smash 64 and Melee, but from what I've noticed of watching Isai and watching Mango is that Mango seems like he can mindgame harder/play just smart in general, catch habits intelligently, an mix it up very well, whereas Isai seems, to me, anyways (anyone who knows the games better can correct me) to have simply mastered the technical parts of the game (spacing, combos, etc).
Like when I watch Isai play 64, it could just be from ignorance and lack of knowledge about the game, but to me it never seems like he's making intense reads, it seems more like he's just covering options perfectly, and comboing like perfectly. Whereas when I watch Mango, he seems like he's just in his opponents head, and gets hard reads more often.
Isai definitely wins more, though, lol.
Although it's more likely than not that I don't know either game well at all and I'm just stupid haha