I have not personally been to the Frozen dessert party-- I've been to the Happily Ever After garden viewing dessert party.
I can tell you, based on research, of the advantages of the Frozen one:
1) Outdoor reserved area with seating (weather-permitting)
2) Includes alcohol for adults
3) Includes a ride on Frozen Ever After post-fireworks viewing (a popular ride that usually needs a fastpass)
4) Some savory foods & desserts too