TF2 is most likely better for multiplayer as you can only play with 3 other players on Borderlands and you have to play off someone's internet so you might have really bad ping often. TF2 also brings more variety than Borderlands and Borderlands gets real boring after you beat so I would suggest buying TF2 over Borderlands.
Tf2. And also, Borderlands doesn't have the best replay value. I tore through it once, and let it go for a while, tried to get back into it to beat it again several times, and can't. TF2 though I can always pop on and get into
TF2; it's an overall great game with developers who (generally) listen to the community and release updates fashionably late on a regular basis (compared to other developers). It is also supported by eGO