One of the best RPG games I played. The game immediately sets you into a dark mood and gets you in for a nice ride. The characters are really well-written with a surpirisingly deep backstory. And the storyline is also well-written too, with unexpected twists. There are also some things that the game will show or tell you, and you may not understand it at first, but time will come that it will all make sense.
As for what version you will play, it's really up to you. The PSP version is good, with new interface, remixed music , difficulty that can be changed anytime, larger character portraits, and so on. However it is disappointingly too easy, but it still doesn't fail to impress players. The PS1 version is also good, but a little slower than PSP. Btw, huge thanks to the people who translated Innocent Sin on the PS1. Back then, only Eternal Punishment had an English release and nobody understood the story. Nonetheless, very good game.