Over the years I've found the ultrasonic cool mist humidifier to work the best. It doesn't get easily clogged up with minerals. It seemed to me that when the mist is cool things in the room get less soggy. And the humidifier lasts for years.
I just bought a new one at Walgreens for $29.99. The directions suggest using distilled water but I haven't done that. There can be a build up of minerals over time, depending on how hard your water is, which I've cleaned out with vinegar. Cleaning out minerals is much easier with the sonic humidifier than the other type.
I've found the ultrasonic one to be much more dependable and to run for longer than warm mist ones. Look for how much water it holds. That determines how long it will run. Mine runs for 20 hours and when I'm having breathing problems I run it almost constantly, monitoring for condensation on windows and walls.
Added info: one of the reasons I use the sonic humidifier is that there are no filters and the system is entirely enclosed thus preventing mold spores to get into the machine. The instructions say to not let water sit in the tank for more than 24 hours to prevent bacteria growth however. Turn it off only when the tank is empty. Sometimes I empty the tank and dry the base instead of waiting for all the water to be used.
I did some research on the Internet and learned that the problem with mold growth when using a humidifier or vaporizer is in the environment and not the unit itself. When the air is too humid mold grows on window sills and walls. One way to prevent this is to turn the humidifier off or lower the amount of humidity entering the air if you see moisture on the windows. Instructions suggest using a hygrometer, available at hardware stores, to measure the level of humidity which should be between 40 and 50%.
I was not aware of the usefulness of monitering the humidity level with the use of a hygrometer. Ive been aware that condensation on windows causes mold growth and that I always had condensation when I used a vaporizer. Often newer houses are so well sealed up (to increase energy efficiency) that there is condensation and mold problems. Mold can negatively affect upper respiratory health.