I've got Fallout 3 and it's awesome! It just depends if you prefer medieval style rpg's or rpgs that take place in a devastated futuristic world, Oblivion's world is larger, but the story isn't as good as fallout's, my advice is for you to get both, since they aren't that expensive, and then ask for your mom to buy you borderlands for xmas, if you like it.
there are many people that would disagree with that. Oblivion's is larger AND it has a better storyline, not to mention that are over 5 different story lines to go with the main one.