Aliens is a better action movie than Alien but I don't think it's necessarily a worthwhile direct comparison as they differ in scope and presentation. Alien is far more unsettling and slow paced, it's a slow burn film. Aliens is more like a freight train, traditional action movie in a sci-fi setting. They're both excellent films and Aliens without question is an amazing action/sci-fi movie but I don't think it necessarily surpasses the original so much as it is just a totally different experience.
tl;dr - Alien is more like sci-fi/horror while Aliens is more like an action/sci-fi film. Alien 3 is a more apt comparison to Alien in my opinion.
Offtopic but I watched Predators last night and was pleasantly surprised at how good it was. Definitely a step in the right direction after those awful AvP movies.