You'll have more options for food at Serenity Bay. I guess I don't know if the food is better at one location or another, though. They're all kind of just mediocre. The desserts are good, though!I've read that the food is better at the Family Beach and the water is deeper.
They have steaks at serenity bay although they aren't very good ones though and aren't cooked well as far as I remember from when I was last there. The podcast team also talked about it on a DCL show and said the same.You'll have more options for food at Serenity Bay. I guess I don't know if the food is better at one location or another, though.
My opinion
The family beaches are bigger and have more to do. In my opinion they will be more crowed at lunch.
Serenity Beach you need to take 2 trams or walk a lot to get there. Also the overall area is smaller than the family beach. But there are no kids. If you wish to do yoga as part of the gym this is where it is done. Also every time I have gone it has had a lot of seaweed.
My recommendation is to try them both out.