There is usually only 1 selection each day. One park is open early, or late, not both. And not more than one park, unless it is considered a very busy time. I have seen, for example, MK opens an hour early and maybe MGM stays open late the same day. But, again, only during a holiday, and not all 4 parks are involved. Usually on any given day one park will have Extra Magic Hour(s) for morning or evening.
Being early risers, we always go to the early hour and plan to park hop when everyone else gets there at 11am. By then, we've hit our favorites and can move on.