The main difference is the capacity of the cartridges. The WGB21B 10'' inch home water purification system has 10'' X 4.5'' cartridges that feature 50,000 gallons of water capacity, while the 20'' whole house cartridges feature around 100,000 gallons of water capacity. The average American household water consumption is approximately 300 gallons per day. Therefore, the cartridges inside of the WGB21B would last 50,000/300=166 days (Around 6 months) on average. If your family consumes more water than 300 gallons per day, you may need a 20'' whole house system to fulfill your needs. You may need to look at your water bill to figure out how much water your family consumes monthly.