Should I get Fallout : New Vegas or Should I get Left 4 Dead 2?

I want fallout new Vegas for it's long suspenseful storyline, a wide variety of thing to do, and my friend has it and says it's good and he also stated that the graphics are pretty good too. I want left for dead 2 for the online co op to play with others, the huge arsenal of weapons, and addictive gameplay. I can only get one at a time for now so I have to choose what I should get first. They are both the same price , at $20 at gamestop but I don't know what would keep me playing for a while. Let me know your thoughts/opinions and a little bit why.