It's been years since I've seen The Spirits Within but I remember being very disappointed with it when I saw it in the theater.
Advent Children was pretty sweet. My only complaint was that it put a little too much focus on Cloud and not enough on the rest of the team. Some of them are only there for like 5 minutes. Besides that it was awesome. I loved the Tifa fight in the church, by far the best of the movie and seeing omni-slash was amazing as well.