You're probably going to need a bigger humidifier than you currently think, as the device will be trying to humidify your entire home, not just the grow room, unless it is tightly sealed. You're also going to need to run it 24/7. I suggest you consult a salesman to help you size a unit capable of boosting the RH% to the 50%-60% range.

As to what kind - How mineral-laden is your water supply?

“Cool mist” or ultrasonic devices atomize the water and force the micro-droplets into the air. Any minerals dissolved in the water will precipitate as a white dust on plants, furniture, and any horizontal surface. The use of pure water prevents that, but some ultrasonic transducers won’t work with it. The mechanical atomization or “buzzing” of the ultrasonic transducer can be a slight problem, but it usually ends up as a mild source of “white noise” that is ignored.

“Warm mist” devices heat the water and add moisture to the air in the form of steam. With those, the dissolved minerals tend to build up on the heating element. Pure water solves that problem. Such devices are essentially silent in their operation.

A third type - the “wick” type of humidifier - is typical for the “whole house” device. Typically a cloth mesh belt or sponge is moved through a water bath (sometimes they just stand in it and capillary action draws up the moisture), and air is blown through the mesh to evaporate it. Mineral buildup will occur on the wick, but it tends to be slow, and generally the wicks are inexpensive and easy to replace, so you need not bother with pure water. The fan may produce a low noise level – probably lower than the cool-mist type of humidifier – but it is usually not an issue.