It really depends on who you talk to as to what answer you'll get. We were first told to use cold air humidifiers, and then warm....it changes constantly. What really cracked me up was when we mentioned one to the pediatrician's office "we were told to use cold air humidifiers" the response we'd get was, "Oh - don't use those, we haven't told people to use those in years..." I responded, "It was last week!"
That being said, we've always had good luck with cool mist humidifiers. We figured with our first daughter getting a croupy cough, they said to take her outside in the "cool night air" and that hasn't changed, so that's what we went with.
Good luck!