9:1 Baluns are useful between 450 ohm balanced lines for center fed Zepps and G5RV antennas where the impedance at the end of the feedline is 450 ohms (1/2 wavelength feedline), or high impedance feeds for log periodic beams.
9:1 Ununs are useful between coax and long wires with load impedances near 450 ohms. Shortwave listener antennas also can benefit with the 9:1 unun and it will provide a better match than 4:1 ununs – this means quieter noise levels and stronger signals so you can hear distance stations easier.