A softener belongs on the line where water enters the house, after the main shutoff and before the water heater. In Houston that is often a garage, a utility closet or the side of the house. We look at where the water comes in, where a drain and a power outlet are within reach, and where you can get a bag of salt to the tank without a struggle.
Indoors, the unit is protected from sun and weather. Outdoors, the pipes are insulated and the equipment is set on a level base. Either way the install has isolation valves and a bypass, so the house keeps water when the system is serviced, and outdoor taps and sprinklers can stay on untreated water if you prefer.