Hi Amy:
This is a great question. You can probably ask 20 different panelists this question and somehow end up getting 40 different answers.
The good news is that I'm always happy to jump in with my opinion. To me, Epcot's
IllumiNations: Reflections of Earth is the top firework show. It's a more traditional firework show with minimal, but poignant narrative. My family enjoys the message and music of IllumiNations. We feel the show provides a wonderful time to reflect on your day and vacation. You are also more free to move about during IllumiNations, in case you have to make a speedy exit for any reason.
Fantasmic, at Hollywood Studios, is a thrilling show. It's much more theatrical than IllumiNations, so if you feel your family would prefer a more structured experience, you may want to consider Fantasmic.
My family would opt for IllumiNations. But if you could squeeze in both, both shows are worth your time.
Enjoy your vacation.