Buying, of course. Streaming can be taken away from you anytime the artist decides to (see Taylor Swift and the Spotify-drama) and I'm not here for that.
The new, young streaming generation doesn't appreciate albums as a whole but is rather chasing the hit singles, investing no big amounts of time to focus on an album. It's a shame.
I rather own the file cause I like to do edits like making a remix and music videos. you cant do that when you only stream the song you need the actual file