B.D.
There are two kinds of humidifiers - cool or warm mist. We have used both and this is what I can tell you.
Warm mist - mineral deposits build up on the element if you have hard water like we do in the North. If you have the humidifier near an exterior wall in the winter, condensation will form and mold will result on the wall.
Cool Mist there are actually two kinds - those with fans and ultrasonic. We tried the fan kind but our house is cooler in the winter to begin with so having the fan run to disperse the moisture just made the room feel colder. We have used the ultrasonic ever since. Ours runs overnight 365 days a year. We do not put a lot of money into them (maybe $25) and they typically last a few years . I basically look for a cheap one that does not require any special filters etc. We clean it just with bleach water on a regular basis.