I've talked myself out of buying an FM-enabled Master System / Mark III many times because while I think it'd be cool to have, it just doesn't sound superior enough to justify the cost for me. For Phantasy Star I think it's cool how it makes the game sound more like its sequel, and how it makes a game that already felt sort of between generations feel that much more high-tech for the time, but the PSG (original) sound is awfully charming. I'd play through it with FM sound... that's the intended experience. I'd just listen to the PSG OST later on and enjoy it for what it is.