alien: a wash, really. the SE's added cocoon scene is cool, but doesn't fit with the later movies and does break the tension (as giger and scott said)
aliens: theatrical, without a doubt. only one added scene actually adds much to the movie (the "ripley's daughter" scene sets up the newt thing a lot better). the rest, imho, detract. the scene with jorden family at the derelict plays like a bad alien ripoff. showing the colony before the marines gets there detracts from the unknown qualities. and setting up the queen ruins the surprise. and another note: that's aliens theatrical on VHS. james cameron films on a format called "super 35mm" which is actually more accurately represented by FULL screen. the wide-screen DVD is a crop. same with terminator, btw.
alien 3, resurrection: ok, i have to admit, i guess i'm a bad alien fan. i haven't actually seen these yet. i'm vaguely interested in alien 3 SE, but the idea of watching resurrection again kind of makes me gag.