Guard Service companies supply guards that have already been trained, passed a background check, and are paid as an employee of the Service. You will pay a higher price for these types of guards, but they will be worth it if your needs are temporary or if you do not want the additional responsibility of a direct hire. It is much easier to dismiss a guard supplied by a service than it is to fire an employee.
The service company pays all wages and benefits and will usually bill you on a weekly basis. The company will probably require the first weeks payment in advance. Make sure your guards are being paid properly. In some states, you can be held liable if the guard company fails to pay the employee.
In many cases, the guard is an off-duty or retired police officer. On the surface, a trained police officer might seem ideal for the job, and in some cases they are. However, police training often includes such topics as constitutional rights, criminal law, traffic law, drug law, use of force, the penal code, arrest, search and seizure, family violence, patrol procedures, and criminal investigation. Although some of this training would be valuable for security officers as well, the majority of police training is superfluous. Security officers receive training in security-specific issues; risk management, physical security planning, CCTV operation, labor relations, shoplifting deterrence, crisis intervention, and customer service.
Unless your needs are simple, stay with the larger firms -- the training and pay are better. Remember, if the guard service is under-cutting the competition, they are likely under-paying the guard. Paying the guard as well as possible is important if you want them to take your business seriously. A single negative incident could cost a lot more than what is being saved by under-paying the people in charge of protecting your business.