When I last went to WDW they were building the MGM Studios. We took one day and went to Kennedy Space Center. We have Sea World in California but we don't have a place where they launch rockets (well we do over at Vandenburg AFB but I can get thrown in jail for trying to get on base without the proper credentials) so that's why we didn't go to Sea World.
Now, with USO I would probably take time to go there because I do know that they have attractions that I cannot experience at USH so I would like to experience them.
But I would still stay on WDW property and spend the majority of my time there.