I've used both and I found the warm humidifier is better. you can open the cool humidiner and study its structure. I really don't see how it can be as efficient as the warm kind which boils the water and let the steam out. The cool kind just blow the moist air in the filter out of the unit without any heating. It takes much longer to exhust same amount of water in the cool unit than it takes in the hot unit. Of course the hot unit requires a lot of work cleaning regularly to reach the best result.