Best Available or Good?

Hi guys,

I need some clarification in iTunes for macOS. In preferences and playback I can choose at video playback quality good and best available?
What Is the difference? I cant see any difference at Harry Potter 4 for example I always download or stream the HD Version. Internet connection is about 480mb/s so no issue with streaming several titles at the same time.

What do you prefer the at the options "Good" or "best available"?