A calculated measure is being calculated at run-time. A derived measure is being calculated when the cube is being processed = pre-calculated. In Microsoft terms a derived measure is called 'Named Calculation' and is created in the data source view. A derived measure uses standard SQL syntax, a calculated measure uses MDX.
Since the derived measure can also be done in the underlying DWH table, you should consider to create the calculation there. This way it will also be available for any relational reporting accessing the DWH directly and you would not have to recreate the calculation when querying the table.