It depends on the thoroughness of the curtain cleaner. Curtain dry cleaning is a foolproof method to remove deep dirt and dust from curtains as it uses a machine and there is little human error to be made.
On the other hand, curtain steam cleaning has a margin of human error. Steam cleaning curtains is a manual process. It is difficult and time consuming process, requiring the curtain cleaner to pay attention to detail and have the discipline to ensure that all areas of the curtain is sprayed with the chemical solvent and subsequently steamed.