Your car's current setup will dictate if a rear sway bar is needed or will add any impvoement to your ride. For maximum performance, comfort or a compromise between the two, you have to tune the suspension. Understanding the concept of roll stiffness as well as roll force distribution will help you achieve your goals.
From a performance standpoint, a "neutral" handling car is usually the better performing car. This is achieved by using different springs and sway bars.
Example:
If your car currently understeers, a rear sway bar can help you achieve a more neutral balance. However, if your car has a tendency to oversteer, and you do not have a rear sway bar, adding one will not help the situation. In fact, it will probably make it worse.
Most of the time, our cars tend to understeer as people will add stiffer coil springs and leave the stock rear leaf springs. A rear sway bar would be a good way to even things out.
If this is the case with your car, then installing a rear sway bar should show an improvement. As far as what kind of rear bar to purchase, whatever you do, get an adjustable rear bar. This will allow you to tune the rear to get the feel just right for your personal tastes.