Alien was all about the slow build up and the tension, which made is a classic, but when it comes to James Cameron's flawless sequel we get a much faster and scarier movie.
The film is about the lone survivor (ok ... the cat survived too) Lt. Ripley (Sigourney Weaver). She is awaken from cryo-freeze only to met by skepticism when she tells her story about the Aliens. This skepticism starts to dissolve when colonists disappear on LV-426. A team of Marines are sent to investigate, along with Ripley, and what they find is of course the acid spitting Aliens.
This is what a sequel should be. This movie got everything right and it's because Cameron understands plot. He stages the action around what's going on in the story and never falls to a mechanical approach, which makes this movie superior to all the Alien films that came after it and films of this type.
Now, is it better then the first? Well, that's a very hard question, but I would say that Aliens is slightly better. Alien is an amazing movie, don't get me wrong, and it really set the template for other films in the franchise and of its kind. Cameron then came in, took that template and evolved it and the result was a much more adrenaline filled film that sees one of the best performances given by Weaver.
I strongly recommend this movie and the first one to everyone. They are both true classics.