What are the major differences between Views Datasource and Views data export?
Views Datasource can be thought as providing a view format that takes over the whole page; each submodule you enabled (JSON/RDF/XHTML/XML) provides a format that it outputs the view as a specified document type without your site theme.

Views Data Export can be thought of providing an attached exportable format of some complementary view format (e.g. a Page view formatted as a table that as an exportable CSV file); when enabled, it provides a new view display that you can make available at a specified path and attached to existing view as a button which can be configured to be a downloadable attachment.

There is some overlap (e.g. both could be used to produce an XML-producing custom path), but only Views Datasource provides a JSON (unless you use some other contrib module), while Views Data Export has other options (e.g. batching support) that make it better suited for large data exports.