I'm assuming you have forced hot air or hydro-air heat.
I'd stay away from the mist type that spray water into the airstream. Our builder installed one of these and it was a pain to get right, get the flow adjusted, etc. Once it was set up, it did its job, but we have very high mineral water (not 'hard' though) and we started to see scale downstream of the unit as well as a very fine dust in the house.
I replaced it with an Aprilaire bypass unit which we like much better. Since it is an evaporative unit, dust is not that bad. Maintenance is a semi-annual rinse and clearing the solenoid screen as necessary. Going on 8+ years.
w