My 9 yo boy absolutely loves Islands of Adventure. However, as a previous poster has pointed out, the minimum height on some of the major rides is 54". When we last visited, in Oct last year, he was almost 9 and not quite at the magic 54" so couldn't ride Hulk or Dragons Challenge and he was severely disappointed.
Having said that, with the advantage of WWOHP, I would definitely recommend IOA over universal if they can only do one of the parks.