There’s definitely differences in the catalogs. If there’s a particular genre that isn’t well represented on YouTube Music that is well covered on Spotify. Maybe they just got YT Premium as a family subscription because the kids are on YouTube all the time. I personally can’t imagine switching to a service that doesn’t have a digital locker for me to upload my existing library like YouTube Music does (though I’m still mad at the uploads being less integrated than they were in GPM).