with fallout 3 don't rush the main story mission otherwise you'll find that you'll beat the game just as fast as Fable 2..
Both games have short main stories but the side quests in both games make them go longer and longer
Fable its kinda like a fairy tale setting meets GTA.. while fallout 3 is like oblivion IV meets First person shooter
Fable II definitely needs the HDD install the game and menu's loads much better.. fallout 3 has some minor loading improvements but the disk accessing is already quick on that anyway so you can just run that from the game disk.