Ah, but see, you're missing parts of the picture.
Lunar goes Lunar 1 (Good Game) -> Lunar 2 (Better Game) -> Dragon Song (Terrible Game). So, starting from a solid game we have one good game and one bad game. 50/50.
Grandia goes Grandia 1 (Flawed experimental game) -> Grandia 2 (Good Game) -> Grandia XTREME (near-plotless dungeon crawler slog with about two steps forward, one back relative to Grandia II's gameplay) -> Grandia 3 (rips XTREME'S gameplay, makes it balanced, then marries it to an offensively bad plot). So we start from a mediocre game then have a 33% chance of good, 33% chance of tedious, and 33% chance of the worst plot ever written (with some gameplay). I can't say I really like those odds at all, so I'd definitely rather gamble on more Lunar.