Which do you think is more of a classic? I think Alien was more of an entertaining blockbuster, but I think Blade Runner is more of a long term classic in that it is a film you think about and want to revisit from time to time.
I saw BR not too long after it came out and didn't love it. Saw it again and liked it, and then really liked it. What is it about a movie that can get under your skin whether you like it at the time or not? I think that is one definition of art, maybe not classic films but definitely art.
I will see Prometheus for sure, but good or not, we won't know how it will last for another 20 years or so!