I don't know if there is a difference in quality. I do know that if you 'own' a streaming movie you really don't OWN it. I'll buy a physical copy of a movie or music any day.
You might own the DVD but not the content. If you owned the content then you could make multiple copies of it but since you don't own it the MPAA would frown on that. I hated those commercials that said "Own it today". Yeah right.