Sara,
When EXACTLY in December were you planning to go? That makes a difference in the answer.
The full week before Christmas week through New Years is the highest cost time at the Fort and its also the busiest. Those periods require a reservation made at least 9 months in advance IMO and hopefully sooner. If you can come in the earlier half of December, its lower costs, less crowded, and just about the same camper decorations around the Fort. The parks are decorated for Christmas and the weather is generally better too.
FSU Ed and eastatlantaguys gave descriptions of the site types you asked about. Let me fill in - the Premium sites are the most expensive, widest/longest, and most numerous types of sites at the Fort. They are loops 400-1400 and are 18 feet wide paved concrete at least 50 feet long with no tent/sand pad. They are designed for the biggest campers/rv's and are easy to back into. They have a square picnic table and charcoal grill with a lid.
Preferred and Fulls are identical in amenities and only differ in location. Preferreds are loops 100-300 and have 12 foot wide concrete drives about 35-40 feet long with a tent/sand pad at the back. Preferred loops are up close to Bay Lake, Pioneer Hall, the boat docks, and a little bit more "in the action".
Full loops are 1600-1900 and are identical in size/shape/design to the Preferreds but are set back in the woods in one of the most quiet section of the Fort. Some sites back up to the canals. For folks that prefer peace and quiet, these are the sites for you.
Premiums are most expensive, Preferreds are next most expensive, then Fulls (with the Tent/Partial sites being the cheapeast). All site types have power, water, cabletv/internet, and sewer (no sewer in Tent/Partial). All loops are served by an internal bus system to get to the Trading Posts and external bus stops at either end of the Fort.
So what site type to get depends on how much you want to spend, do you want "peace and quiet" or "in the action", and how big your camper/rv is (the bigger should lean toward Premiums).
Hope this helps.
Bama Ed