I suggest getting Demon's Souls solely because it might go out of print pretty fast like many Atlus published game.
I found Fallout 3 to be the better game though. Demon's Souls isn't all that great, imo. Out of the 8 levels that I played, I only like 3 and two of them can be beaten in 20 minutes or less. Boss battles are pretty disappointing. I like the boss scales and all, but fighting them is kind of boring - tower knight was the only one I found fun and challenging. Speaking of challenge, the game isn't as hard as the reviewers put it out to be; you just have to move slow and not rush in. Story isn't remotely interesting, and the combat system is just average.
Sorry for my small rant.