I LOVE the Fable series, and I would say Fable III is the better of the two. Fable: Lost Chapters is still the best game (I don't really count the original Fable and Lost Chapters as separate games since Lost Chapters is just an expansion of the original, but since the expansions are awesome, I specifically picked Lost Chapters, lol), though, so if you haven't played that one yet, I strongly advice you to. Sure the graphics are not as good as it's successors, but it's a great game with a great story and tons of things to do. Fable III has a slight slower start than II with the huge amount of cut-scenes at the very beginning detailing your brother, the king's harsh rulings and so on, but if you just stick through the castle-bit, and don't get too confused by the "Road to Rule", which is basically just the way you level up in the game, then the rest of the game is awesome. The final boss could have been more challenging, though.
I, too, thought the idea of playing the ruler would be kinda stupid for a Fable game, but it honestly works. You have to go among the people, gather support from the people, storm the castle once you have enough support and take over the throne and face a whole new challenge once you do, as your brother wasn't a tyrant for no good reason...