Well, now i've played fallout 3 and watched fable 2 gameplay, you're better off with Fallout 3. You get:
-more immersive gameplay, everything is realistic and touching.EX. Some friendly npc was questioned by her father (the overseer, big boss in there, which I didn't recognize) and an officer. I thought both were officers, so I whacked their heads with a baseball bat. Well, I feel bad and stupid now for killing my friend's father. This you won't get in Fable2
-more hours for the buck
-more cussing (abundant as hell

)
-more mature content (not in terms how many insenuated fannies you see, in terms of relations, how things work)
-just more