I really really love them both but I said Universal Studios. Not sure why but I just love the surroundings, the rides, everything. Plus its the original park. I just love it!!
My vote goes to IoA. Prior to it being built, Universal was a 1-day trip of a longer Orlando theme park tour. Since IoA, Universal has become my home away from home. I'm on my 4th year with an annual pass, and lately have been making 2 4day trips a year down there. For me it's the combination of amazing thrill rides, best in the buisness theming, great food, and nice employees. I can't wait to go back next month for my first HHN experiance