If you plan to rent a porta-potty (portable toilet), the most important item to consider is the reliability of the servicing company. If your porta-potties start to fill, or leak, or stink, or need service, you want a company that will drop everything and come make the required adjustments. [please scroll down]
Ask for references. Ask your friends. Go to a construction jobsite and ask the foreman.
The better companies are proud of their service. The bad companies hide behind their low prices and fast talk.