I saw the complexity in Spirits Within, and I usually watch it when they play it on TV, but the reason I think many don't see Spirits Within as a good movie, is because it has absolutely NOTHING to do with the way Final Fantasy worlds are governed. It was just a movie, movie like any other, with a rather simple theme.
Also, if you like to figure out stuff, boy you'll have a field day with this. So many things aren't explained for you in this one, I doubt you'll be able to get them. For example Materia, the importance of specific moves, anything about Jenova and its history, who the hell Aeris is, etc.
Too many things to count.
Game>Movie is my suggestion.