I haven't played Fable 2. However, they're probably on the same level. Both are great games, and both sort of sandbox type run-around do whatever type games. They're sort of in the same genre. You can't really compare the two. However, if you must, then I'd say Fallout (then again, I haven't played Fable so I can't really state my opinion).
Although I haven't played Fable 2, I heard it was kind of short. You could probably complete it in less than 10 hours. Fallout, however, is at least like 30.