To be honest I played the L4d2 demo and have put a ton of time into Borderlands. After playing the L4D2 demo I bought it immedietly, the new changes are just huge game changers and it's going to so much fun playing the new infected.
Borderlands I got 2 50s on and I really don't have anything left to do untill they make us pay for more DLC, sure I have awesome gear but it really isn't that much of a varation. Killing the same bandits throughout the game got a little boring. Diablo 2 held me in for years because of just the amount of varation of monsters/enviroments/weapons (that had different colors/looks/feels/radically different stats, the stats in Borderlands are all the same on each weapon but some weapons have different numbers on those stats, or have modifiers etc or some might have one of the few elemental effects that you see over and over) and endgame and difficulties. Borderlands just seems really easy, and there is pretty much no endgame or boss to go after, and the enviroments are pretty much the same throughout the game untill the end. You pretty much see the same monsters throught the game too, and the guns even though randomized are pretty much similar in look/stats which got kind of boring. So right now Borderlands is just sitting there in my gameslist till it gives me something to do, amazing game untill you beat it.
L4D2 will probably last you a lot longer, L4D lasted me sooo many hours and I still play it all the time, Versus is just to much fun/competetive, and they fixed so many things in it it's seriously going to be a whole new game. They removed the repetetive weapons, new infected including more than just SI, there is clown zombies and things. There is a bunch of new levels that have really interactive enviroments that really immerse you into the game. Like at the finale at one of the levels there is a mic you can yell into and it amplifies your voice and makes it echo.
Only people I found getting bored of L4D were people that only played campaign, I mean.. seriously how can you not like versus once you become good at playing the infected, it's just so much fun. Valve also adds a ton of DLC to keep it interesting.
I will probably put thousands of hours into L4d2, versus the 200 hours I put into Borderlands.
Your choice though, you might be less hardcore and the 200 hours of Borderlands might last you longer, just make sure you have friends to play with, because I always seemed to outlevel my friends and they wouldn't come on and I wouldnt be able to play and would have to catch them up.