It seems that in UK, Tidal HiFi Family is GBP29.99 while Amazon Music HD Family is GBP19.99. That's GBP10 saving per month. After clarifying with the user, it turns out that he included the 90-day free trial into the calculation, i.e. saving 29.99 * 3 originally for Tidal, got a once-off discount for a being a Prime member, and excluded the cost of Prime membership (which is useful for other purposes).
From another forum discussion, I also see someone preferring the Amazon music library to Tidal.
From another forum discussion, I also see someone preferring the Amazon music library to Tidal.