Buy park-to-park if Harry Potter is why you came
The Hogwarts Express runs only between this park and Islands of Adventure, and boarding it in either direction requires a park-to-park ticket. On a one-park ticket you can still walk into Diagon Alley and ride Escape from Gringotts; what you lose is the train, which is a real attraction with a different show in each direction, and the Hogsmeade half of the story entirely.