Well best pick for me would be Borderlands. Left 4 Dead 2 aint so fun in my opinion if you want to seriously play a game. Borderlands has better gameplay and graphic which dint really get me bored of doing quest and finding good weapons.
Left 4 Dead 2 is just one of those time-killing multiplayer game which just not worth it unless you play online with a bunch of friends to kill time or with random strangers who doesnt gives a damn about saving your a s s.
So Borderlands for me in that case plus it supports multiplayer as well for faster quest completion
. Though I only played the Xbox360 version with split screen but its fun