IMHO, it's not worth it to do the dinner package in early December. Whether you do the dinner package or not, you will still have to arrive early for Fantasmic, just not quite as early with the dinner package. The seating for the dinner package is all the way over at one side of the theatre, so you're more likely to get better seats without it. Plus, if you do the dinner package, you each have to have the fixed price meal that includes appetizer, entree, and dessert. I don't know about you, but the portions are so huge at all the Disney restaurants that I find it to be a waste of money to order all that food and throw at least half of it away, so I always share.
In early December, you shouldn't have any trouble getting into Fantasmic an hour or so before showtime with general admission seats. With the Fantasmic Dinner Package, it is recommended you arrive 45 minutes prior to show time. I believe at 30 minutes before showtime those reserved seats that have not been taken, are opened up to the public. So, for only a 15 - 30 minute difference in the arrival time with the dinner package, plus the extra cost involved, plus the area in which the seats are located, it is not worth it, IMHO.
Even over the 4th of July weekend this summer we had no problems getting seats for Fantasmic and only got there 1 1/2 hours early. We would have had to arrive at least 1 hour early with the dinner package for the holiday weekend. Personally, I don't see what the big draw of the dinner package is. Doesn't seem like it saves much time at all and costs a small fortune. Now, if you were planning on eating at one of those restaurants anyway and would actually order and eat all that food, then it's probably a good value for you.