High efficiency units are cost efficient and optimised to use as little water and salt as possible during regeneration.

If you have 3 to 6 bathrooms in your house then you probably have high water usage. A high flower unit with a faster flow rate will meet the needs of a large house.

An electronic control unit requires a nearby power-source to run features such as the on-board computer and LED read-outs

If noise during regeneration is a problem for you, look for units which are quiet running.

Many water softeners are single tank so when they're in cleaning mode, your supply of soft water is interrupted.  Twin-tank operate so when one tank is cleaning, the other takes over to give you continuous soft water. 

This salt is sold in blocks the size of a house brick, generally 2 blocks per bag. They are generally easier to handle and pour.