I actually don't like the U/S ones. I think the cool mist are all ultrasonic types too. I prefer the warm mist ones they don't drop the temps any and they do a better job of humidifing as they actually put water vapour into the air not water mist. They also are not prone to bacteria growth as they boil water to work not vibrate it so the humidity released is basically sterile they don't need to be cleaned daily.