I don't mind arguments, so long as people will actually argue logically about the objective merits of the two games. Typically what happens is these threads turn into flames where people insult each other back and forth rather than actually debating, but I don't think that this is inevitable, just common.
It's entirely fine to like Fallout 3 more than New Vegas, it's also entirely fine to like Superman 64 or E.T. for the Atari better than New Vegas. That really doesn't address the issue of which is the better game overall. I typically try to stick to defending my premise that New Vegas is better for xyz reason, and avoid insulting those that disagree (a good policy for any debate topic, really).
My mind is entirely open to being changed about any topic, assuming someone puts forth an argument good enough. And I'm not really asserting that Fallout 3 svcks, just that it's a good game that fell short of being one of the best games of all time because Bethesda made a couple of huge, huge mistakes that just can't be ignored. That isn't me flaming/trolling, it's giving my view on the game (and my reasons, if requested). I'd think Bethesda 'wants' to hear what their major mistakes have been, so they don't repeat them.