As I see it...
Fable 2: I liked Fable 1, it made me somewhat angry that they only came though with a fraction of what they promised, but I think that with Fable 2 they're actually going to be able to do it. The replayablility to make it worth the cash to buy for myself is in the multiplayer and non-linear open world. Also, I like RPGs in general.
Force Unleashed: I love physics candy, especially with destructible environments and this game looks like it has all the physics I could ever want to play with. The replay value will probably simply be in my constant messing around with stuff to find new and interesting ways to kill enemies. Also I'm a Star Wars fan.
So it's been totally up in the air for me.