It all depends on the height of your kids.. my 8 and almost 6 year old prefers IOA over USF.
My 8 year old adores coasters, and the Spierman ride can't be beat. Hulk & DD are two amazing coasters.
If they aren't tall enough for the coasters, there is still Spidey, Jurassic Park, the Toon Lagoon, water area, Dr. Doom (depending on height), the playland in Jurassic park, the Discovery Center in Jurassic Park, Storm Force Acceletron, dudley do right, Posiedon show (very cool!), there are a couple of other things, but I can't remember off the top of my head
USF has MIB, T3, and a few other things that appeal to my kids.
My kids prefer IOA, but that is my kids. They are thrill ride nuts, and tall for their ages.