Which Park Should I Do With My Rollercoaster Loving 9-year-old?

So I'm taking my 9-year-old to Orlando for a few days this summer before he goes off to spend some time with his grandparents. I've decided to do one day at Universal and another day at the water park or Disneyworld before we go back home. Which park would be better for my 9-year-old who loves thrill rides? We're from Southern CA and go to Universal Studios Hollywood on a regular.