While these two approaches look similar, there are some important differences.
When using a hierarchy structure you can choose between a form or a hierarchy uniqueness. This is not possible when using branches. The branch uniqueness will be limited only to each branch scope as the hierarchy uniqueness cannot be applied.
When using child forms, you can have only one child form per each parent (see linking forms). Using branches, you can have multiple branches for a single form. Using branches though, you can go down only one level (it is not possible to add a branch within another branch), while child forms can have a hierarchy of maximum 5 forms.
Child forms and branches are rendered in a different way on the mobile app.See how to add a child entry.
The mobile app bookmarks feature only works with child forms.
Branch entries are not downloaded to the mobile app, only hierarchy entries are. Learn more.