Hi, Lisa!
I am so happy to help!
Your question is a good one, as
Disney's Hollywood Studios and
Epcot are so very different from each other; each one is wonderful, but each one offers something unlike the other.
If your little princess is a fan of all of the Princesses, then consider adding dining reservations in Epcot. There, you'll find wonderful dining, with Princesses, at Akershus Royal Banquet Hall, in the Norway Pavilion. You can also find various Disney characters throughout the World Showcase, such as Jasmine and Aladdin in Morocco, Snow White in Germany, Belle and Beast in France, and quite a few more. There are also some truly fabulous dining opportunities at Epcot, both in the World Showcase as well as in Future World. Check out Les Chefs de France, where you might spot Remy, or Coral Reef Restaurant at The Seas with Nemo & Friends Pavilion, where you'll dine while gazing at thousands of fish in the 5.7 million gallon tank. IllumiNations: Reflections of Earth is a wonderful way to round out the night, too.
On the other hand, Disney's Hollywood Studios is spectacular, too. You can see Beauty and the Beast-Live on Stage, enjoy Toy Story Mania!, The Twilight Zone Tower of Terror (DD should be tall enough), Muppet*Vision 3-D and Voyage of the Little Mermaid, not to mention Fantasmic! In fact, should you choose HS, try to book a Fantasmic! Dining Package at Mama Melrose's Ristorante Italiano. (Let me know if you're on the Dining Plan, too. There's a secret I'd love to tell you!)
If it were my family, I'd visit Disney's Hollywood Studios with a 5-year-old princess. However, I know that you would still be really happy with a visit to Epcot, too.
Please send more questions about your upcoming plans; I love to help!