Alien 3

Aliens 4

I admit Alien builds up suspense a lot better, but then again it had the advantage of being the 1st one. Ridley Scott did a great work on building this incredible ominous atmosphere, and fill it with a clever deconstruction of the "body snatcher" cliché, but heavy tone of the film is severely diluted outside of a cinema or projection room, unless you're watching it alone at night in complete darkness. Most of its entertainment value is lost after watching it for the first time, and after that the movie is mainly worth for technical details and plot innovation. Additionally it will also appear to be a lot more boring, unless you're realy into that sort of thing.

Aliens, on the other hand, maxes up the entertainment by spicing it with a lot of great action scenes and plenty of crowning moments of awesome. This might be a classic audience catering move, but surprisingly it doesn't wash away the metaphoric aspects of the original plot, as the main real-life paralels established in the 1st movie are still present and fairly well done, although a bit overshadowed by the action itself (it's not a perfect movie after all). Aliens also does make use of a few streamlined characters for the sake of audience emotion exploitation (like Burke or Newt), but if you're contemplating a movie as both an artistic and entertainment experience, you have to learn to tollerate these, as sometimes they're necessary gimmicks to capture the general public attention, as cheesy as some details may sound.

All in all the main point in favor of Aliens is that it's the kind of movie you can watch anytime, with anyone, due to being a fairly well-directed blend between horror, suspense and action. Pick both movies and try watching them on a saturday night with your regular non-movie intelectual friends. What will likely happen is that they will get bored after 30 minutes of Alien hide-and-seek, but will scream "F*CK YEAH PVT. VASQUEZ!" the second you show 'em the Aliens dvd box.