Depends, presuming warm mist vaporizer is the same as a humidifier (I don't know). Cool mist is more prone to grow and disperse mold or bacteria if you don't keep it well cleaned, since it doesn't boil the water like a humidifier does. Steam coming out of a humidifier can scald, though. We went with a humidifier and keep it up out of DD's reach.
Re: Cool/Warm Mist Vaporizer?
I heard Cool mist was better too!