What is the main difference between prevention and long-term management of asthma?

Probably the Same. Prevention and long-term control includes avoiding triggers of asthma, such as allergens, smoking and infections. Sometimes daily medicines such as inhaled bronchodilators, steroids, or oral leukotriene inhibitors singularly or in combination are necessary to prevent asthma.For this type of therapy, consult your doctor.
Not much. The prevention of an attack is one of the main goals of long term therapy for asthma. Avoiding triggers, minimizing inflammation, and treating allergies (if present), treating reflux, exercise, are all part and parcel of both.