I think that you should call the CiviCRM API directly. This allows you direct access to the CiviCRM system.
If you use Drupal Web Services you still would need to wrap the services around CiviCRM API calls (or direct calls if needed).
So Drupal Web Services just adds additional complexity to something that the CiviCRM API does an excellent job of all ready. Just use the CiviCRM API services.