Alien is one of the theatrical cuts that I prefer to the extended version. The major additional scene creates a huge continuity issue, since it adds an element to the xenomorph's life cycle that contradicts what is established in Aliens. Also, most of the snippets added don't flesh out all that much, and if anything, strain the pacing a bit.
While I certainly enjoy the added footage in Aliens, it still suffers from the same pacing issues. Cameron knew how to make a theatrical cut perfectly balanced. The special editions of his films are excellent supplements, but to me, his release versions are superior (Abyss may be an exception). T2 is an obvious one. Theatrical cut is a perfect film. All the edits made the film better. The deleted footage is certainly fun to see though.
Now with Alien 3, that's an entirely different beast all together. While I still, in spite of it's flaws, greatly enjoyed the film in it's theatrical form, the assembly cut was a revelation. Nearly every hole was patched up. This was an example of proper expansion.
Resurrection...didn't notice much of a difference between cuts.