The Whirly was used because we didn't have a choice - once the Wessex was available, the Whirlwind soon went out of service.
The definition of "soon" is totally subjective, Crab. The HAR10 actually continued in service alongside the Wessex for more than twenty years. I fondly look back on my time flying them (and prior to that, a short time under them, getting wet, on what some would consider the "wrong" end of the winch cable).