You need to compare first cost, including installation. Spread oout cost over life of the system. And you need to compare average annual operating cost. Electrical costs are based on cents per kilowatt-hour (kWh). KWh is determined by kW ( which is amps x volts to simplify) times the number of operating hours. You can find average annual cooling hours for your city. A good sales person should be able to help. As the first answer stated, it depends on the space.
Central air is more expensive than window air conditioners when the setting is small. Window air conditioners are economical when it comes to a small room because of the cost of energy involved. However, central air conditioners are less expensive for big rooms since they are 3.0 more energy efficient than window air conditioners in that case.