The difference between a resonator banjo and an open-back banjo lies in their construction and sound projection. Resonator banjos have a closed back with a detachable wooden resonator that enhances the instrument's volume and tonal projection, making them well-suited for playing in a band or on stage. On the other hand, open-back banjos lack the resonator, resulting in a mellower and softer sound, making them popular for solo playing and traditional folk music.