The website is not clear on the differences between the models-- besides price, that is. Travel Berkey? Royal Berkey? Imperial? What is actually sets them apart? Can someone offer some opinions? My details: Live in the northwest USA, but in the dry part. Will be used for a family of two, with two dogs. Interested not only in daily use, but having a reliable filter for emergencies when city water is unavailable. Of course, we would like to keep the purchase as cheap as possible. Any thoughts?