I have only had a Serenity Beach cabana, not a Family Beach cabana, and it was last month during my Magic cruise. I loved the cabana and would book one again on either beach in an instant. Given a choice between the two, I would choose Serenity Bay. It was so peaceful and comfortable. I liked that it was an 18+ beach just because that meant there wouldn't be any screaming babies or tantruming kids (I love children, but in my cabana I want peace) at any time. I also liked being close to the Serenity Bay barbeque, which was better imo than the barbeque on the family side of the island. I had lunch in my cabana, and it was very easy to just walk with my food from the barbeque back to my cabana. My cabana was also an easy walk to the adult bike rental. And of course, the price is hundreds less on Serenity Bay, so that is a big bonus, as well.
For me personally, the only drawback of the Serenity Beach cabana is that the adult beach is an unprotected beach, unlike the Family Beach. That makes it prettier to look at, which is nice while you're lounging around, but I am afraid of swimming in beach waves (had a close call once), so I won't do more than wade on an unprotected beach. I'm not a huge beach swimmer at all, though, because of that experience, so it isn't a huge factor for me. I noticed there were a lot of sea shells on the Serenity Bay beach, which was a positive, too.