So youtube-dl shows that YouTube only uses Opus, m4a and vorbis containers. So how do these YouTube to MP3 converters get the MP3 then? Do they transcode from a lossy format? Isn't it better to get the Opus file, as it's the best quality?
It would definitely bet better to get the Opus format since Opus makes better use of the available bitrate and transcoding is a lossy process anyway.
However, MP3 is a format ubiquitously supported by listening devices and thus is a "convenience" format.
Additionally, Youtube changes its preferred format around all of the time. If your library management can deal with different formats, you will likely be best off by taking what youtube-dl delivers (even better would be to use the original upload in whatever format it was but I think that may be only available to the uploader themselves).
Since Youtube is transcoding themselves, the quality of the Opus file might already be compromised more than its nominal bitrate would suggest.