There are SBP ( small by-pass ) LBP ( large by-pass ) and Fan Power
Choosing one all depends on the Furnace for ideal operation, if your furnace is a single stage a by-pass is great, if your furnace is 2-stage you WILL FOR SURE need a power assist. When I say power assist I mean there is actually a miniature fan blowing/forcing the humidity into the duct work system. Two stage furnace usually run on low stage 80% of the heating season and they just don't have the CFM are movement to pull proper air across the humidifier pad that is Moist, hahahaha moist !!!
I have installed Aprilaire, Honeywell, Bryant, Brauburn.
Hands down Aprilaire is the Best, not only is it the best but there all priced the same so choose the aprilaire.
Also there is Manual option and Automatic option, manual means as the temperature changes outside you must walk to the control that is mounted downstairs and adjust it, Automatic is just that, you set it once ehhh possibly twice if your totally new to it and you leave it alone for how ever many years your in the home.
Automatic versions have a sensor that you place outside, the sensor takes a temperature sends it to the control, control does a % calculation and determines what ideal for the setting of where you want it.