AAC has widespread support, Opus does not. Unless you know what you're doing (have to use the Ogg container and a .ogg extension and use Android >= 6.0), you c

AAC has widespread support, Opus does not. Unless you know what you're doing (have to use the Ogg container and a .ogg extension and use Android >= 6.0), you can't fill an Android device with Opus files and play them back like you can AAC or MP3. I don't believe you can play them back at all on iOS. Mobile devices also often have a hardware codec that allows them to save on battery somewhat, when playing other codecs. Plus desktop support is limited. Browsers support it but widespread desktop apps like iTunes generally don't.