Yes, you are right. So the question is which model is better: a tree model (with nodes as flat tuples) or JSON model with nodes being either collections or comb

Yes, you are right. So the question is which model is better: a tree model (with nodes as flat tuples) or JSON model with nodes being either collections or combinations (with arbitrary nesting). It seems that JSON is more general while XML requires you to map the reality into a tree representation. It is somewhat similar to the relationship between the hierarchical data model and the relational data model.