Just in case you haven't found out yet... 999 came first. It's a DS game. It was reprinted to coincide with the release of the 3DS game, though, so more people could play it. "9 Hours, 9 Persons, 9 Doors" is the original release (the cover shows all the characters), while "Zero Escape: 999" is the rerelease (and shows two hands reaching out). Definitely play it! But be warned, the second game does spoil the first one quite a bit.