Well the documentary material in the Quadrilogy version is excellent and essential. If I had to choose, I would probably have to stick with the Quadrilogy version, but I have both editions anyway.
Ive got the Quadrilogy Set, The Legacy Set, The Legacy Documentary, The Alien Saga Documentary, and the Alien CAV Laserdisc Box set... and I wont be parting with any of them!
The Legacy 5th disc is the hour long ALIEN Legacy documentary, which is entirely about the making of the first film. Its a very entertaining look at the production of that film and is really essential to any ALIEN collection. I can't imagine why FOX would leave it off the Quadrilogy (maybe they didn't want that set to be any more intimidating than it already is) but it should have been on there, and it should have been included with this 2 disc re-issue. I'll never part with my copy of it.
It doesn't show up in their regular inventory when you do an "Alien" search. I found it by clicking on the other release of "Alien" and then clicking on Ridley Scott to find all of his titles, and the the original verison shows up.
It arrived today, and it is the version with the (I think) better Ridley commentary and isolated music score.