Personally, I think you might want to consider staying at one of the UO resorts that offers Express Pass. Although you can't use it at most of the Harry Potter stuff (with the exception of the Dragon Challenge and Flight of the Hippogriff), you might be surprised at how much you enjoy having easy access to all the rest of the attractions. You can utilize early entry to ride on the Harry Potter stuff and then the Express Pass the rest of the day for everything else.

Another thing to consider is staying one night at the Express Pass resort, that way you can still use the Express Pass for two days (the day of arrival and checking out). St. Augustine is not that far from UO; you could easily spend a full morning and afternoon at the parks and then head over to the coast.