Not that it's really possible (or fair) to compare the two games, in my opinion Fable III will still be my favorite game by the end of the year.
Now, I don't have anything against Assassins Creed Brotherhood, because I will definitely be getting that game shortly after it comes out. Although, mostly for the multiplayer. (I'm not too excited for the single player, then again, I never really am when it comes to AC games.
I adored Assassins Creed II, unlike the first one which i shelved after only playing it half way through, and I'm sure I'll enjoy Brotherhood just as much. But I've always liked the Fable series much more than the AC series.