Colleen,
I have never been a big fan of the Fantasmic dinner package. While I know it is free, I have always thought that the seating location for the show was less than optimal. The one real perk was getting to avoid the longer main line.
Now that participants of this package have to use the same line as everyone else, the only real perk is that you have a somewhat "reserved" seat for the show, which is especially good if you think that you won't be able to get in line early enough to get a seat, or you are here during the busiest times of the year when the show reaches capacity most nights.
Since you are coming in September, I would recommend that you not bother with the dinner package. Just go to whatever dining you prefer and get in line early enough to ensure that you get a good seat for Fantasmic. I would recommend either
50s Prime Time Cafe (table service) or Sunset Ranch Market (quick service) for both good food and proximity to the show entrance.
Have a magical (and delicious) vacation!