As stated above, IOA is mainly for thrill-seekers. It is a great park to experience. There are 2-3 rides that will get you really wet--you may want to plan for that.
Universal Studios (USF) is our personal favorite. It has no coasters, but does have a lot of fun attractions such as Back to the Future, Men in Black, Jaws, King Kong, Twister, Earthquake, ET, and an awesome 3-D show called T-2 (based on the Terminator movies). There are also a lot of shows that are very entertaining.
I guess it just depends on what your interests are. We personally spend about 1/2 day at IOA (not into coasters, but LOVE spiderman) and about 1 1/2 days at USF.
One more thing: Depending on what kind of passes you have, you can park hop between IOA and USF. So you can always play it by ear on which park you want to spend the most time at. They're right next to each other, so it's not an inconvenience to walk back and forth or to change your mind on which park you want to go to any given day.