You can create a smartlist that shows tracks present in one list but not on the other, just add these 2 rules:
Playlists - is any - Top 500 Jaren 70
Playlists - is not any - Spotify nog niet
Or vice versa.
You can type the equivalent expression in the search/filter box as well. Select the "Top 500 Jaren 70" playlist, then type this to hide the tracks present in the other playlist:
-playlists="Spotify nog niet"