Hello, Nikki!
You've got two great choices in these two nighttime shows, both Epcot's
IllumiNations: Reflections of Earth and DIsney Hollywood Studios'
Fantasmic!
Both shows are interesting, but very different from each other. While IllumiNations is really inspiring, if I had to choose between the two, it would be Fantasmic! This is a show unlike any other you've seen, where favorite bits of Disney movies are projected onto water screens, many characters from Disney films make appearances and beautiful, lit boats sail some of the characters right up in front of the audience prior to the firework finale and Sorcerer Mickey's triumph. (Now a few moments of this show are dark, when some villains appear; I see from your earlier questions that you have a 4 -year-old, so be sure to talk about how Mickey will surely succeed if things get a little too intense prior, or during, showtime.) You'll be so glad that you selected this one!
So that you're not disappointed, I'd head to the show about an hour before showtime, in order to get a good seat. While it's possible that you can arrive later and still be seated, several years ago, we arrived 30 min. prior to showtime and had to stand. Best to avoid that; grab a snack and a drink, head into the theater and enjoy the atmosphere of the crowd until show time.
Hope that helps! Send more questions as you think of them! You know I love to help!