I'm not an evangelist, but it's easier to load kids, and when kids are over three years old even the roomiest of trailers get cramped. In warm weather, riding on the back is really fun for the kids.

The trailer is better for foul weather, ice, and frigid cold, and really little kids. Cheaper, as you say. More of a hassle.

Neither option is bad. But I don't regret the cargo bike when I remember how excited the kids are to ride it.