That is a really tough question to answer. Diagon Alley in US is completely amazing, but I feel like the tour through Hogwarts in IOA is equally amazing. In US, you will have only one ride, whereas you will have two, including Forbidden Journey, which, in my opinion, is the best ride anywhere, in IOA. If you were able to tour Hogwarts in California, I would say go to US to see Diagon Alley. If you are really big Harry Potter fans, I really feel like you should get the two park ticket to see both of the areas.