if you have:
the first release of Alien (with the audio comm, and the isolate alternate score
the Alien legacy DVD
The Alien Quadrilogy box set
the Alien Saga (image Ent DVD)
and the Alien Evolution uncut version,
so you have all about Alien
but if you are interested in movie trailers, this dvd includes as extras, a lot of 20th century fox trailers, with trailers for Alien and Aliens, not seen on the alien quadrilogy set.
about the Alien Saga DVD, apart from the documentary, it also includes :
original screen test footage
Alien featurette (1979)
Aliens: Behind the Scenes featurette (1986)
Grunts in Space: Aliens featurette (1986)
profiles of Sigourney Weaver and James Cameron (1986)
4 theatrical trailers
review here : alien saga review