ive only played them a couple times each

fable 2 is great but theres some things they changed from the first one that would have been better if they had been left

fallout 3 is a challenging game and will probly give you more playtime.

fallout is probly the way to go but if you dont like it just return it and get fable or do what my friend did: get fable play it a bunch then return it for fallout