I prefer Movie Magic, though Final Draft is also a fine choice.
I haven't had luck with free downloads, which I have looked into for my students (a few of them, not all of them by any means...and I don't recall names/or don't want to risk naming names; such a coward, but I am also computer challenged...so the free stuff may just have not worked for me).
Next semester for my Advanced Feature Script course I am requiring Movie Magic, yet allowing Final Draft for the handful of students who already own that.
Academic Superstore has the best price for each that I have seen - $89.95 for Movie Magic, including shipping and handling. (Think Final Draft is up around $150 or so....)