I took my then four (turning 5 the next weeK) year old to both parks.
SHe liked them both, but she is very adventourous.
In USF, she liked 1) Hanna Barbera, 2) Men In Black 3) ET, 4) the play area 4) Wild West Show, 5) Barney 6) Back to the Future. 7) Woody Woodpecker coaster
She wasn't crazy about 1) JAWS --as long as she wasn't on the side she was fine 2) King Kong --she cried the first time, and then the second time just laughed.. 3) T2-- first time she didn't like it, but then went on it and laughed! 4) Make up show--she cried! but as soon as I told it was fake, she did ok, but she didn't want me to volunteer though!
Earthquake and Twister were just ok.. no problems
In IOA, she liked
1) spiderman! 2) Storm Force 3) Dudley (she was tall enough) 4) JPRA (was scared the first time but after that she was fine!) 5) Popeye 6) Camp Jurassic 7) Pterdon Flyers 8) Flying Unicorn 9) All of Suess Landing 10) Sinbad show
She wasn't too crazy about Posiedon the first time, especially the last battle scene at the end.
I hope that is helpful to you. Both parks are really good, but it goes down the how adventourous your kids are. My son loves IOA, so I anticipate us spending more time at IOA than at USF.