While wall units cost more to install, window units are harder to seal, block your view outdoors, block incoming light, and prevent the use of the window for natural cooling and ventilation. Central AC systems have a number of advantages. They are out of the way, quiet, and convenient. If you already have a central heating system that uses forced air, you may be able to use the same ducts. Finally, for cooling large homes or homes in especially warm climates, central AC systems are more energy-efficient.
Heat pumps, although more expensive, provide heating in addition to cooling.
If your home does not need a heating system or if you already have a satisfactory oil or gas heating system, a heat pump probably doesn't make economic sense. If your home currently uses electric resistance heating, however, it may make sense to investigate a heat pump to satisfy your cooling needs as well as replacing your electric heating system. No matter which type of AC system you choose, sizing it to your home and cooling needs is important. More so than with heating systems, the cost of air conditioning systems rises significantly with larger equipment. Don't let a salesperson convince you to buy a system that is larger than you need.