Opus is better than MP3 in every way. File size is either better or the same, and audio is better even at lower bitrates. But realistically, most streaming services don’t provide HD audio, so it really doesn’t even matter.

249 webm  audio only      2 │    1.58MiB  49k https │ audio only         opus        49k 48k low, webm_dash
250 webm  audio only      2 │    2.09MiB  65k https │ audio only         opus        65k 48k low, webm_dash
251 webm  audio only      2 │    4.14MiB 128k https │ audio only         opus       128k 48k medium, webm_dash
233 mp4   audio only        │                 m3u8  │ audio only         unknown             Default
234 mp4   audio only        │                 m3u8  │ audio only         unknown             Default
140 m4a   audio only      2 │    4.20MiB 130k https │ audio only         mp4a.40.2  130k 44k medium, m4a_dash

This is YouTube music, which generally serves the split audio from a YouTube video as a song. Most of them I checked either don’t have audio above 130Kbps or don’t even provide MP3/Opus anyways.