New Vegas by a mile. That said FO3 has Liberty Prime. You can get your fix of him via youtube though.
FO3 is oblivion with guns.
New Vegas feels much more like an actual fallout game. It has opposing factions that are mutually exclusive. I.e. joining one closes off the quest chain for another. You can't do everything in the game with a single character like you can in Oblivion/FO3.
FO3 is the Corky of the Fallout series, just atrocious and full of stupidity like everything Beth does. NV is almost ruined itself by being based on such a terrible engine and design, fortunately it has much better writing and mechanics to make up for it. --- "The only thing that comes to a sleeping man is dreams."