I would assume in a direct comparison - streaming a couple gb from a data center is probably much more environmentally friendly than moving a physical disk across the country.
It would depend on the specifics of each implementation, but shipping 10 discs total over a year is likely to be more efficient than to keep streaming infrastructure up and running for that one movie.
Obviously, the same streaming infrastructure would be used for other content, but how many (unpopular) shows are only watched a few times a year yet they take valuable infrastructure capacity?
And then we get into lack of popularity leading into obsolecence, and we are now talking a much wider topic.
Disk space is cheap. I would assume that they don't really take infrastructure capacity.