Left 4 Dead, and Fallout 3 is basically Oblivion with guns. So if you want to try out guns or like them more, get Fallout 3, and if you prefer fantasy, go with Oblivion. Left 4 Dead is made by Valve, which makes it a great game already. Valve aren't the type of developer to walk away from a game, they'll update it for a large period of time.
Team Fortress 2 (Orange Box) is an example of Valve updating their games, at least on the PC. They're also trying to get most, if not all, of those updates for 360. PLUS, Left 4 Dead is a big title on the 360 for Valve, so they will hopefully update it quickly, and there is talks about 2 new maps for L4D, and adding 2 existing ones to multiplayer. That alone will keep people playing for months, if not years.