While Crash might not be able to choose (and I know that choosing between two great movies can be an unenviable position), I find Aliens all-around a more solid film.
Alien does a lot of things right, and I rate it pretty highly, but Aliens changes directions beautifully and it works out to it's benefits. It may not have the entirely same feel of suspense that the first has, but it more than makes up for it.
