I am going to do a bit of transitive properties here. - Talking about family = family plan to me.

So for both services we are talking 6 devices streaming their own accounts for $20 a month.

At the point in our argument there is no huge difference between the two to be honest. Same price for the same services for Music. They both provide good audio and playlist function. (lossless audio on normal speakers and car speakers doesn't do much unless we are talking about $$$ cars/home audio and at that point this conversation isn't happening)

The big difference for me is that we have apple mobile devices. (I tried Android some time ago but didn't like that each company had their own skin and updates depended on how each company felt at the time. The nexus I had was great and was straight from google no extra crap but even that had a 2-3 year span though so no different than Apple for me at the time)

With Apple one (6 devices for apple music), every family member gets Apple TV+, Apple Music, and Apple arcade. For me this put it over the fence. Kids get more games I don't have to pay for and we get an extra streaming service I don't have to pay extra for won it for me.

Essentially extra value won it for me. Not just the music service.