Air conditioning is a must when it comes to beating the summer heat, but while central air is significantly more efficient than window units, it will cost you much more in the long run.
Central air is convenient, but it's used to cool an entire house, even when you're only occupying a certain area of the home. This means quite a bit of energy and cooling power is being wasted. In contrast, a single unit is only used when someone is in a room, so it's much more energy efficient.
Generally speaking, users will keep the quiet central A/C running when they leave the house, but since window units generate a lot of noise, they're not as easy to forget to shut off. According to the Energy Information Administration, the convenience also means that central air users are much more likely to use their A/C regardless of the outside temperature.
On the other hand, window unit owners tend to only use their A/C on the hottest days. This means less energy output over time. A 2001 analysis by the EIA says that air conditioning accounts for more than 15 percent of the energy use of the average home, so it's a big area for savings. Choose window units over central air and it'll cost you less to stay cool.