Oddly, most of my friends rate the first film higher than the second. But personally I prefer the second. I totally appreciate the "crash bang wallop" element; Alien is much more spookier than it's sequel. But Aliens is just relentless as a "chase" movie and leaves you feeling completely drained by the end of it's 2 hours plus running time. Besides Aliens has one of the best film scripts ever (imho!).
Aliens is the one I'd watch with friends for a laugh. "One of the best film scripts ever" is a just a little OTT I think, but it's horribly quotable.