Although there are some similarities between the two, there are also some very important differences.
Roman chamomile oil is derived from Chamaemelum Nobile (a roman chamomile plant), while German chamomile oil is derived from Matricaria chamomilla.
Within the practice of aromatherapy, Roman chamomile oil is used to lower stress and improve mood while German chamomile oil is used to ease skin conditions.
Roman chamomile oil, also known as English chamomile oil, is a little cheaper than the other. However, German chamomile is used more often.