A cool mist humidifier requires the vaporization of water. Since the water is not heated, the water can be contaminated with bacteria if you do not change the water frequently. If you have kids around, the cool mist humidifier is better than warm mist because warm ones can be very hot.
Ultrasonic humidifiers simply causes vibration to water, and they also use non-heated water as well. It all depends on your preference as to which one you prefer. Choose the humidifier that suits your needs.
Here is an ultrasonic humidifier that has both warm and cool settings:
SPT SU-4010 Ultrasonic Dual-Mist Warm/Cool Humidifier with Ion Exchange Filter
