I personally think it's both.
So, we have two basic themes: the primal savagery of man - his more or less concealed human desire to be unbounded by society, morality and the limitations of his mortal body and the fear of the unknow, of death from without, a danger hiding in the darkness beyond our sight.
Like Paul Church said in the Aliens Labyrinth comic:
"In their hearts, all men would like to be like the Alien: hideously strong, unchained by conscience, charged by the black heart of the cosmos to go forth and annihilate.".
You can translate it as a fascination with sociopathy.
Now if you look at sociopaths, you can see they are both human and inhuman: they are the core of human insticnts - selfish, egocentrical, arrogant. Yet they lack traits that are widely considered as those defining human beings: empathy, morality, remorse. So they are monsters, inhumans.
They both repel and fascinate. There's a weird ambiguity here, and I think the Alien is such an ambiguous symbol: it is both human and inhuman, alive and dead, sexual and asexual (lack of gender). It represents both the unknown within (instincts, desires, destructive tendencies) - the Giger approach - and the uknown without (other people, threat of death, fear of sexuality) - the Lovecraft approach.
You can both identify with and fear a sociopath. Be the agressor and the victim at the same time. It doesn't have to be sociopaths, that's just my kink. Same can be applied to forces of nature, sexuality, etc. That's why the Alien can represent both the inside and outside world of the human mind.
I probably sound a bit chaotic, but that's my two cents.