Fostair is a combination med using a LABA (long acting beta agonist and an ICS (Inhaled Corticosteroid). Trimbow is three medications: a LABA, a LAMA (Long Acting Muscarinic Agent) and an ICS. So Fostair is two meds; Trimbow is three. Which is best depends on you and your needs. Some people get along just fine without the LAMA; others don't. It's not a question os which is best; it's a question of which works for you.