They're at both, and they're awesome.
Here's how it went last year. My dad and I ate a wonderful dinner at Mythos in front of the window looking at Hulk. Then we walked outside and the show started. The back of Mythos is a great place to watch. There's a story to the show, a battle takes place. Fireworks shoot across the lagoon back and forth. Lots of fireworks shoot up from the middle. Music plays (orchestra), and the fireworks are synced to it. There's a quiet part in the middle where a circle of fireworks on tall poles sway back and forth. Then a grand finale with intense music.
As you are walking out of IOA, you see the end of the USF show in the distance. Don't have the details, but there's a story to the IOA show with the battles. Maybe someone will post it. It's one of those hidden things you would never know about. The USF show takes place in the middle of the lagoon. Their fireworks seem to shoot higher, and seem to be bigger. While the IOA ones shoot across the lagoon and are more specialized. The patterns of the IOA fireworks are unique. You don't see them at the regular shows at your home town. They do weird things, dance in the sky. Hard to explain, but they're really good.
Eating dinner at Mythos, walking out to the back, watching the show, and then seeing the end of the USF show as you leave, is a great end to your day.