There are many features that drive the choice between one glass bong or pipe and another. Looks, budget, and color changing features are just some of the factors that come into play. For me, more than anything else, I look for a well placed carb.
A carb allows you to quickly clear out the chamber of your water pipe or bong and improve on the quality of your smoking experience. A small carb limits the air that can help you to clear the chamber while one that is too big or poorly positioned will lead to awkward and inefficient use. Be sure to test out the feel, look and practicality of the carb on your water pipe.
In addition, a carb that is well positioned will allow you to better clean you glass bong or pipe and provide for a cleaner chamber to smoke from. This also contributes more to the smoking experience and allows for a more enjoyable and better smoke than many other features.