I agree that the laserdisc colors are the best overall for Alien, although I think you can give yourself a little latitude here and there. And I really like the idea of having the original 70mm audio track as an option, as I missed that particular laserdisc.
Regarding Aliens, I think that you should definitely start with the theatrical because I think that it would be worthwhile preservation (was the AC-3 track on that last laserdisc edition also the 70mm track?). That said, I think it would be a great idea to re-create the ABC cut.