Avatar was great, I don't think I have ever seen anything that makes another world come to life like it did except for Peter Jackson's Lord of the Rings trilogy. It didn't fall into the trap of having too many digital effects in every scene, a mistake George Lucas made with his Star Wars prequels. Visually the world of Pandora was great, believeable except for the floating rocks which were never explained and totally in contraction to not only the laws of physics, but the technology present elsewhere in the story. I liked the alien biology, inspired by undersea creatures on our world. The best way to sum it up is as Aliens meets Titanic (both of which I think are much better films from the same director).
My problem with Avatar is that if it wasn't a visual spectacular I would have felt I had seen it all before. Essentially the plot is the same as Dune, and while the characters are interesting and it is easy enough to empathise with their motives, I had no real contenection to them emotionally like I did, say with The Lord of the Rings, which I mentioned earlier, or the Harry Potter movies. The biggest problem however was that all the ideas in the story I've read hundreds of times before in science fiction novels. Still, really enjoyable, don't get me wrong about that, I just don't think I'll race out and watch it again anytime soon.
District 9 however had a premise that initially turned me away. If aliens can come all the way to Earth they won't be refugees, they'll conquer us as is always a flaw in movies like Independence Day and its like. However as soon as I started watching it I realised it was a farce on South Africa's Apartheid era. The slums with all its the crime and subculture was exactly how I remembered my experiences with that kind of living that I encountered in Kenya many years ago, and I found myself laughing at little elements in scenes that my friends didn't pick up on, such as the operations of the Nigerian warlords. It felt like a story Douglas Adams would have wrote if he were a white South African.
The story was overly graphically violent, which has never appealled to me in movies, although I know this is a personal taste. The story was strong and made me laugh with its mockumentary style. It was refressing to see a film shot in Johannesburg, as I'm so over watching science fiction movies set in New York, Los Angeles or Washington DC (and District 9 even makes a joke about this). The themes too were so much more poignent and original than Avatar, and the characters felt more real. What District 9 had over Avatar were ideas that I hadn't seen before, in fiction or movies, and so for me made it the much better movie for this genre.
Regardless, I'd recommend both films to those who like their sci fi and their action.