Diferences between: migrate_d2d and migrate
From Drupal-toDrupal data migration (migrate_d2d) module page:


  migrate_d2d provides a framework based on the Migrate API


So the diference is - migrate is an API module for migrations, and migrate_d2d uses it to perform task like yours. Either way you will use migrate, only difference is - do you feel you need to write what authors of migrate_d2d already wrote? Or maybe you want to use or modify their work to save some time?