I just finished playing the demo for Heavy Rain, and I'm kind of on the fence about it. Graphics are astoundingly good, even though the uncanny valley effect is still tangible, voice-acting is equally good, the atmosphere is nice and gritty and even though QTEs appear to be almost 100% of the gameplay they're well-implemented; I want to see the story and how it ends, but I'm not sure if the game has any replay value at all, because once you've seen the ending of a thriller the element of surprise is irrevocably lost... that's why I'm not sure if this game's more than a good rental.
As for FF13, I'm looking forward to it as well, but to give you any actual advice on which game to get we'll all have to wait until it's finally out, don't we? ... which kind of renders the OP moot.