You could certainly do either way. It is super easy to walk between parks, so you can go back and forth several times daily with park-to-park tickets. You would also then get to ride the Hogwarts Express train between parks, which is a different experience each way. Park-to-park also would give you a little cushion in case a ride is experiencing technical difficulties during your day in that park, or in case one park is unusually busy (though express passes mitigate these issues). It's up to you whether the benefits are worth it.
Also if you just do one hotel night, you may want to plan the water rides accordingly so you will have access to change after.