I like fallout 3 much more than borderlands. Borderlands is fun, sure, but I really only like to play it with other people. Fallout 3 was awesome because you could play it in so many ways. (If you take fighting style and karma into account, you could play a total of 18 ways, assuming you only chose one karma path and one weapon skill.) VATS is just beautiful, and I love a lot of the weapons on fallout 3 as well.
However, for Borderlands if you have online capabilities (or are getting it for the computer) I guess that would probably be a better choice. I only have Borderlands for my non internet connected xbox 360 (with only 3 controllers, no less).
Even if I could play with more people, though, I'd still play Fallout 3 more.