Both really.... went when DS was 3, 4, and 5.
Hollywood Studios - DS liked meeting all the characters, not found in the other parks. Liked dancing with Agent Oso at the Hollywood and Vine breakfast. Fantasmic! I guess Fantasmic could scare some children (so I heard) but DS had a great time. At age 4 and 5, he gained more interest in the shows -- Lights Motors Action, and Indiana Jones. And Toy Story of course... and the movie ride. Did I mention he went on Tower of Terror? What I like about Hollywood Studios is things seem closer together so less walking from one attraction to the other.
Epcot - Lots of walking is the biggest downside. Didn't mind it so much when DS was 3 and in a stroller, the other years we had to slow down a bit. Garden Grill dinner was a big hit with Chip and Dale there. Turtle Talk with Crush is a must do. DS liked all the performers in Epcot more so at age 4 and 5 -- there were some street performers in UK, the drummers in Japan, the Morocco band, etc... And slightly off Epcot, at the Boardwalk, he really liked the performers there - juggling, balancing, etc.. Epcot has characters too.... very nice to meet Belle and the Beast in France!
I left out a lot of stuff.... but between the 2 parks.... Hollywood Studios is DS's favorite by far.