Soundiiz is supporting both YouTube and YouTube Music platforms.
The main difference is how about we are dealing with matching to these platforms and how we are retrieving metadatas.
When transferring a playlist or tracks to YouTube, we are matching to most popular videos in the YouTube catalog.
When transferring a playlist or tracks to YouTube Music, we are matching as much as possible with audio only videos in the YouTube Music catalog.
Also, on Soundiiz, if you are connecting the YouTube Music platform, in addition to retrieve the name of the video, we are also able to retrieve the artist name. That could be very useful if you want to export to another music service or to a file (CSV, TXT...).
And finally, be aware that YouTube and YouTube Music are sharing the exact same user library. That means a playlist created under YouTube account is also available on your YouTube Music account, as long as you are using the same Google account.