Window/Wall units are very popular and inexpensive. They are portable and are easily installed in windows. Wall units are more permanent. Both units need to be sealed during the winter if the window units are not taken out. In both cases it is better to seal them from the inside than the outside. Some units will void the warranty if you cover them from the outside.
The best way to seal them from the inside is to take off the front cover, fold the wire inside the unit and put a piece of plastic over the front and put the front cover back on. Then trim the excess plastic around the unit. If the unit is in the window, the place where the two sashes meet should also be sealed.
Take a piece of foam rubber and slip it between the two sashes at the top of the bottom sash. Then tape it air tight. It is good to note that the air control knob should be closed during the winter.
There are advantages to using window units for your cooling needs. Versatility of these units is superior to any other type of cooling system. This is apparent when cooling your bedroom at night instead of the entire home. If the unit in your bedroom failed, it would take less than an hour to remove it from the window and replace it with a unit from another room. If you compare the cost of purchase of these units to cool your entire home to central cooling systems, you'll see there is not that much difference. The real difference is when the systems need maintenance or replacement.
The disadvantages to window/wall units are inconvenience, though their purpose certainly out weighs it. Most manufacturers try to eliminate the most common complaint about window units and that is noise. Their appearance and sound gives occupants the feeling that these units do not belong.
Unlike central systems that are a lot quieter and appear to be part of the structure, window units appear and sound intrusive. Not only from the interior but also the exterior. There is no mistaking a gray metal box extruding from a window. Another concern is dry rot if the unit is left in the window for several years. Moisture and dirt builds up on the sill and degrades the paint and eventually rots the wood. It is recommended that every other year the unit should be taken out, the sill cleaned and painted.
You should also inspect around the perimeter of through the wall units annually, both interior and exterior, and caulk and paint where necessary. It is the inconvenience that window units have that makes central systems more favored over them. This does not mean that central systems are as cost effective or versatile as window/wall units.