Picking out an air conditioner can be a difficult proposition, since there are so many different options out there. You don't need to stick with a central unit, and an alternative solution might be an even better fit for your situation. For example, here are two of the most popular alternative air conditioning methods: portable and evaporative AC's.
Portable Air Conditioners
One of the most common alternative air conditioning solutions is to get a simple portable unit. Though they aren't as small and portable as space heaters, they can still be moved if necessary. They don't require the same extensive ventilation infrastructure as central systems, instead relying on an individual exhaust system. You will need to set this system up, but it can be very easy if you have a window available.
As far as efficiency is concerned, portable units don't really offer the same level of power that you would get from other units. They will be a little less efficient in terms of pure output, but they can actually be more efficient when you look at the situation on a room-by-room basis.
If you have a central system that is cooling your home, then you might be wasting a lot of power on cooling rooms that you aren't using. With a portable unit, you are only cooling what you need, which means that very little of the output is wasted.
Ultimately, portable units are a great choice if you need to cool small areas of your home or need to cool multiple areas at different times. They are not the best choice if you instead need to cool a large area all at once or if you already have a comprehensive ventilation system in place.
Evaporative Air Conditioners
You could also consider an evaporative air conditioner, which operates quite differently from other air conditioners. The general idea is that an evaporative air conditioner will raise the humidity of air in your home, while simultaneously lowering the temperature. This will create a very cool and pleasant environment, making you feel comfortable.
In some ways, this works similarly to sweat, where moisture on your skin will make you feel a lot cooler than the air around you.
However, these units aren't very effective when they aren't able to dramatically lower the temperature and raise the humidity. In other words, they aren't a good choice if you live somewhere that has naturally high humidity or that has relatively cool air in general. In those cases, a more traditional air conditioner service would likely be a better bet.