Though as most said, they are both just too classic. I could never decide, one day it may be one , the next another. They are truly the only two movies, that I have to say are just complely tied.
I think that Alien gave more of that, feeling of being alone, and alienated in it self. Which was incredible. Both had great character development, and story, but just potrayed in other ways.
And to Evil Twin as for the cliche characters of the marines you pointed out, when the film came out, they were not cliche, most of those movies from then out copied Aliens in terms of characters and diversity. Aliens was the first real powerful sci fi action/military movie. And it has stuck in most minds, and writers of other movies have copied it making it cliche.
But both of them are just amazing. In my top movies of all time for sure.