Choose peak flow meters when the question fits its direct job: it measure peak expiratory flow during a forceful exhalation, providing a technique-dependent airflow value that can be compared with a personal best and clinician-defined asthma action-plan zones; steam inhalers instead produce heated humidified air intended for short-term comfort around nasal congestion.
The deciding issue is different mechanisms. For peak flow meters, examine peak expiratory flow and personal best; for steam inhalers, examine humidified air and thermal exposure. Feature lists become useful after the use case is specific.