When I tested run Whisper, Penguin and AC side by side years ago, Whisper is the quietest, next is Penguin, and the loudest is AC. But silence comes with a trade off. Whisper is the worst self starter because it has the weakest pump. I had to finger kick start it often if I didn’t clean the impeller weekly. That’s too much and a safety risks to my fish when I go away, so I abandoned Whisper. AC is the best self starter, and Penguin comes next.
Now with a new line of HOBs with in tank motor, they can be powerful, quiet and reliable restarter at the same time. I have not tested run them though, purely theoretical assumptions.
I have a trick to make HOBs quieter without trading off restarting reliability. I replaced my Penguin 350 impellers with smaller 200 when the original worn out. It pumps less water, but quieter without trade off in restart reliability since the motor is the same. Slower flow rate will not turn the biowheels but I don’t use nor ever installed the biowheels anyway. Slower flow rate actually provides better mechanical filtration by clogging up the filter pads more thoroughly thereby delaying filter cleaning.