Make sure the unit has enough resin to treat all the water you and your family will use. As of this writing, the average usage per day, per person (including children), for inside the house is 87 gallons. You should also be shown two or three ways to initiate recharging the unit.
The oldest way is by a time clock, i.e., your water usage is calculated and the frequency of recharging programmed into the timer. On the appointed day, at the appointed hour, the unit recharges. If all went as calculated, ok. If you were gone -- too bad -- you just wasted salt and water. If you had extra company -- too bad -- you ran out of soft water. You must pick a unit that will treat one days supply of water and still have about 40% of the resin in the recharged state. This will provide you with the most efficiency for salt and regeneration water.
A second way to initiate recharge is by electronic sensing. By electronically checking the resin, these units can determine when the resin needs to be recharged -- this is a great help when your water hardness changes, when you have extra company or when you are gone for a few days. These 'sensor' units can save you up to 42% of your salt and recharge water as well as keep you in soft water when you have extra guests.
A third way to initiate recharge is by using a meter. These units have a meter installed in the water line and simply measure how many gallons of water you actually used. The unit is set according to your water hardness, and will recharge when the gallons used approach exhaustion of the resin bed, saving you a high percentage of your recharge salt and water.
Many variations of these methods are on the market. Some use computers to calculate in advance, when to recharge the unit; some have two resin beds (tanks), and switch back and forth between the two, keeping you in soft water all the time, at the highest efficiency. These systems are most effective in high hardness waters, i.e., over 10-12 GPG, and over 4 people in the family. Low hardness water and smaller families do not require the extra expense of these options.