Could someone please confirm whether the audio commentary for Alien on the Alien Quadrilogy set is a different one from those on the original DVD release and the 2003 version.

I can confirm that it is different. The legacy release had a Ridley Scott only commentary, where as the quadrilogy release had a combined commentary from many different people (recorded in difference sessions.) I can't remember if the Scott commentary in the combined commentary is just pieces from the original or not.

http://www.dvdcompare.net/comparisons/film.php?fid=6

This is slightly off topic, but Ridley Scott's commentary for the 2003 release was one of the few commentaries I ever listened to, and thought it was amazing.

I'm pretty sure Ridley and Sigourney are recorded together on the Quadrilogy, so it would be an all-new track.

I own both discs and can confirm the two tracks are completely different.

The ensemble track on the Quadrilogy disc is great (Stanton's comment related to the hypersleep wardrobe remains the single funniest thing I've heard in a commentary), but Scott's solo track on the original disc is also top notch if only that he's given more room to discuss the film more fully.

Yep, I just listened to this commentary again a couple of days ago, and the recording is clearly with Sigourney present.

The reason I asked is that on the 2003 director's cut release, there is a commentary track with Scott and Weaver together.