Why only a half day?
That would be a tough decision. What are you interested in? More rides or more shows?
I would go to the Universal web-site and see what is in each park and go from there.
We stayed for 10 days last trip, so you question is tough to get in the mindset for. Here goes. I would pick IOA. The rides I would try to catch are Hulk, Spiderman, Kong, JP River Adventure, HP Forbidden Journey, Dueling Dragons. Eat lunch at Mythos and done.
I would recommend Studios over IOA because Diagon Alley is worth its weight in admission. Plus, the queue lines weren't very long for Men In Black and The Mummy which are right next door to DA. Krustyland is right across the way also if you are a Simpsons fan, the theming there is fantastic.
IOA - Most rides are scattered around the Circle. Start at Marvel for Hulk and Spiderman, Next for Kong, Jurassic Park River Adventure, Harry Potter, Dragon Challenge, then Cat in the Hat and your back at Port of Entry and the Park Exit.