Star Trek model: this is also, kind of, the Star Wars model. Aliens are seen as potential friends or at least potential neighbors. Granted the Star Trek model depends on existentialism, reducing all alien races to a few existential characteristics (so that all Vulcans are logical, all Klingons are warrior-like, etc.). Granted, too, the function of Star Trek aliens is to allow human issues to be discussed that, for reasons of political correctness, can't be discussed about humans. Hence, even though all the humans in Star Trek are rational and vaguely agnostic, they are still able to discuss religion with Bajorans and with Worf.
Star Wars follows this model in that the Star Wars universe is peopled by aliens working side by side with no one much remarking on the fact. I prefer the Star Trek version, simply because I loathe (as in detest to my heart's core) Ewoks. Star Trek aliens may be existentialistic but at least they aren't cute!
All in all, this model is remarkably inclusive. It is one of the better side-effects of liberal humanism. Aliens, however problematic they prove at first, can be loved and understood in the long run.
Interestingly enough, however, even in Star Trek, the scariest enemies are bug-like creatures. The Borg is Starfleet's main enemy, but the Borg are still comprehensible. When Star Trek wanted to created a worse enemy than the Borg, what did they do? They brought in the bugs.
Which leads us to model 2: Independence Day. In model 2, the aliens are buggy and evil. They can't be reasoned with. They don't seem to have reasons, just superior technology (that can, nevertheless, be brought to a standstill by a mild, little earth-created computer virus. Yeah, right.) And well, thank goodness for Will Smith, I say.
The X-Files model: In the X-Files model, the aliens are big and bad and buggy, BUT they aren't the real enemy. The real enemy is the government that hasn't told us, the American people, about the big, bad, buggy aliens. In fact, in X-Files, the aliens, or rather the existence of aliens, represent for Mulder belief and hope. The problem is the humans who get in the way of that belief and hope. Men in Black is this model turned on its head. (Yes, the government isn't going to tell you, but they aren't going to tell you for your own sake.)
Personally, I don't know. I think it is possible aliens are out there, although I don't spend a lot of time tidying my apartment for a possible visit. Like any good Star Trek fan, my own sci-fi universe follows the many-aliens-functioning-together-in-the-same-universe scenario (although I leave open the possibility that said alien societies are a tad more complicated than they appear on the surface).
But truthfully, in my heart of hearts, I think Douglas Adams is right. If there are aliens, they don't pay us much mind. They are no more good than your average pompous liberal (such as the Vulcans of Enterprise). And no worse than your average bureaucrat who wants to build a bypass through our galaxy. And if they do make contact, they will be Ferengi wanting to trade. Face it, they won't go looking for the Dalai Lama or George Bush or Al Gore or sincere Hollywood stars or even Queen Elizabeth. They're going to be dialing Donald Trump's number.