I love both... I am not sure which one is better... Valid arguments for each. Also a bit apples to oranges as they are very different genera with Alien being more classic horror while Aliens is more Action Sci-fi.
But... Head to Head... You have to chose only one to watch... And if you don't choose, you are forced to watch Alien 3 and Alien Resurrection Clockwork Orange style... Which one of these two great movies do you choose?