One thing to remember about Fantasmic! is that it's only shown twice a week now - I believe it's on Tuesdays and Thursdays - except for in high season.
Fantasmic! has a food and beverage minimum of $750 and a venue-rental fee of $250 for each of the two patios. One holds up to 40 people, the other holds 40-100 people. Fantasmic! has a higher likelihood of being cancelled due to bad weather, and the backup is one of the indoor event facilities.
Epcot has venue rental fees ranging from $275-$575 depending on the location you pick. The only food and beverage minimum is $22/person. IllumiNations is almost never canceled due to weather. If it rains, you will have your dessert at an indoor event space and then be given ponchos and umbrellas to go out and see the fireworks.
Both parks have a $12.95 viewing fee for each person who doesn't have park admission. There is also a $75 setup fee for sites at both parks, except the upper UK patio in Epcot.