I agree with others, if choice is one day park hopping or 2 days without, I'd do the two days for a more relaxed experience. The only thing you would miss would be the train, which is great for a Harry Potter fan but also don't feel like it is an absolute must do.
If the choice though is just whether or not to get a park-to-park ticket for the days or not, go with the hopper option. The parks are right next to each other so it is very easy to go between them and can almost function as one massive park. Plus, if a ride in one park is down for some maintenance issue one day you would have the chance to catch it the next, or just be able to redo whatever you want.