Wow, there's some great bullshit arguments in this thread and we're only on page 2. Awesome!
Personally I think Borderlands feels more like a shooter AND more like an RPG than Fallout 3.
Mostly because you actually have different classes with unique class-skills that play totally different. Fallout 3 has the same problem Oblivion had; if you played it for a while your character could pretty much learn all skills and be good at everything.
Now some people would argue that Role Playing is all about your role in the unfolding story, but then you would have to agree that Call of Duty is an RPG, too. Which is ofcourse nonsense.
One could also argue that Fallout is an RPG and Borderlands is not because in Fallout you can make choices that (slightly) influence the story while in Borderlands you follow a linear storyline, but then you'd also say that World of Warcraft is not an RPG. Which is also nonsense. (I know there's at least one of you going to say "No, WoW is not an RPG, it's an MMORPG!11" now but honestly, if you do that you should be hit in the balls).
Role Playing isn't just about story, it's also about the role your character can take on in the actual gameplay. Allowing a character to learn all skills destroys any sense of a 'role' in that perspective. As for the world, both games are equally linear. They both point you in the exact direction of the quest (which is something I hate about both), and both games let you choose how you travel there.
I would agree if someone said that Fallout 3 is more of a traditional RPG while Borderlands sort of takes the MMORPG approach to roleplaying, but both are definitely RPG's.
I really can't say which one I prefer though. I totally love the art direction and the whole self-mocking wacky genocidal co-op fun in Borderlands, and I like the atmosphere and story in Fallout 3. Not to mention the level at which it can be modded and allows the community to add content.
I dislike the repetition in Borderlands, but then Fallout 3 is also repetitive as hell, it just sugarcoates it by giving the questgivers different names and a slightly different haircolour.