The model 600 operates as a bypass humidifier; the model 700 has an integral fan. The retail price difference between the two is $150 +/-, although the 700 is easier to install so that might account for the small net cost difference. For what it's worth, I wouldn't install a humidifier unless you find there is a compelling need to do so. If there is, I'd go with a steam humidifier like the Aprilaire model 800.

Thanks very much for your response we’ve been overwhelmed with a number of house projects and there really isn’t great guidance out there. I thought the 700 was more expensive and harder to install, but my husband seemed to think the fan model on the 700 would be preferable. We also want to ensure it’s compatible with the nest learning thermostat and it wasn’t clear if the 600 is. We had considered the 800 steam but were told it would cost 3x more to install and it would likely generate more humidity than needed. Thanks

Where are you located? Does you house project include new windows? Outside air infiltration in the winter tends to lower indoor humidity. If the windows are tighter then you should measure the humidity in the winter to determine if you need a humidifier. I agree with Charles Ross Homes in that you should not install a humidifier unless your humidity level is constantly below the 35-40% range.

Sorry, no help on specific models. Just want to say that we've been pleased with flow-through humidifiers. No way for mold to form or be circulated through the house.