If it's crap ... We'll tell you
Aliens has a more original story and is an improvement on the original Alien film, Avatar is basically Dances With Wolves in space, but everyone knows that!
However, as good a film as Aliens is, it's still your basic haunted house movie with some good production value and effects for the time and some interesting characters, but I'm just mesmerised by the level of detail in the environment of Avatar not to mention the performance capture technology, so I'm going with Avatar as the better film, but that's just me!
Watch Aliens. It's way better.
Aliens, ten times over.
Avatar is the better movie visually. Aliens has a superior plot, better characters, dialogue and no crap in it like Unobtainium.
So Aliens quite easily.
Ghosts of Mars or Halloween?
Aliens, all day and all night.
Seriously? I liked Avatar, but come on. Aliens wins by a landslide, though The Abyss is my favorite of Cameron's works.
Aliens because it had great action and a good storyline.
Avatar just relied on special effects.
Aliensss.......as a horror fanatic Sci-Fi horror has a special place in my heart as does Alien and Aliens =). Avatar does nothing special for me as hyped as it is.