That's a tough question to answer as there are rides at each that we never want to miss - Mummy, MIB, ET (and probably now Simpsons) that are among the must-dos at US; and Spiderman, Ripsaw Falls, Bluto's, Jurassic Park among others at IOA. We generally buy the tickets that allow access to both parks so we can get in at least some of these from each park whenever we go. You might consider that type of ticket along with the express pass to get in as much as you can on your one-day visit.
Dick Taylor