Get L4D2 if you have friends who aren't total douchebags.
L4D2 way more fun if you are playing with people you know.
It does indeed have co-op that forces you to work together, and vs. mode where everyone just pisses all over each other because that didn't get to be the tank. Then there is survival, basically just surviving as long as you can. Then Realism, stuff gets real there, and mutators, which change every once and a while. Also with the AI Director, there is a lot of replay value.
And let's not forget Steam Workshop support!
If you have no friends, or just not interested, then get Fallout. It's an amazing game. I have it.
(i don't have the dlc's because i'm a dunce and didn't buy them so i can't tell you much about the DLC)