I've actually only recently realized that I enjoy USF a lot more than IOA, as I spend a lot more time there whenever I visit UO (Disney parks still top my list by far though). When IOA first opened, I thought it made USF look really bad in comparison. But recently, IOA seems to have been left in need of a really good rehab throughout the park and has lost a lot of that initial "luster" I 'thought' I noticed on my first trips. Ironically, but the more I visit IOA, the more I actually dislike it. I don't know why....it may just be that despite the great themeing, it still has an amusement park feel to it for me in parts. Most will probably disagree with that assessment, but I can't really explain it beyond that. Its still a great park and one that most will probably enjoy.
For me USF is the better park. But, if you want more bang for your buck and love thrill rides, I would agree that IOA is the way to go. I'm sure you would enjoy either.