Important

A student needs to decide which university to attend. One university is close to home and less expensive. The other is far away and costly. She makes a list of reasons: better teachers, more opportunities, future job prospects. After careful thought, she chooses the faraway university because she believes that decision will shape her entire career. It is not an easy choice, but it truly matters. What kind of decision is this for the student? It is an important decision.

Important means having great value, significance, or consequence. Momentous, by contrast, means of great historical or epoch‑defining importance, often a turning point. Important focuses on general significance and worth. Momentous focuses on world‑changing, historic events.