Original Alien film: Should I start with 1979 theatrical or 2003 Director's Cut?

You want to see the oeiginal theatrical release. Even Ridley Scott prefers that version.

http://avp.wikia.com/wiki/Alien_Director%27s_Cut

Scott has stated that he was very pleased with the original theatrical cut of Alien, saying that, "For all intents and purposes, I felt that the original cut of Alien was perfect. I still feel that way", and that the original 1979 theatrical version "remains my version of choice".