Both parks obviously have a lot to offer, and if you're hitting all the Disney parks plus Universal/IOA, I think you're going to have one very happy kid on your hands!
One thing to be aware of: Islands of Adventure has several big thrill rides, such as the Incredible Hulk coaster. A kid takes one look at those and says, "Wow, we have GOT to do that!" But of course, he may not meet the height requirements... not to mention the fact he might well, depending on how well he handles such things, freak out on the ride itself! This is less true, in my opinion, at Disney or at Universal than it is at IOA.
For example, in MGM, it's pretty easy to avoid a kid who might not have the height or temperment to ride Rockin' Roller Coaster... after all, it's at the back of the park, in a separate building, and you might be able to steer him clear so that he doesn't even see it. But with something like Hulk or Dueling Dragons, it's right there, and there's no hiding it!
Richard