The actual functionality between the two is the same. The difference between them is that the Data Echo Framework allows maintenance to be performed across all data types whereas the Data Type Manager limits maintenance to just the one Data Type input by the user on the Data Snap screen.
The primary use case for the Data Type Manager is for users to be able to customize the Data Snap screen with those tables-fields which need to appear as possible selection options for user inputs. The captures below show how this functionality ties together.