Personally, I have no preference.
From a financial perspective, Star Wars would make them more money over a longer term. Unless the next 3 Avatar films are a lot better than the first one was, the storyline and world simply aren't as engaging as Star Wars. I don't see any little boys running around in blue costumes, but I do see plenty of them wanting to be Jedi.
Unfortunately, at the time they made the Avatar deal, I don't think they had any idea that they were actually going to get Star Wars. Lucas surprised the world when he decided to give up control. If the Star Wars deal had gone through first, we'd probably be seeing ground broken on that.