The decision to hire a uniformed security officer or a plain clothes officer should be based on whether it is more important to deter people or to catch them. Keeping people honest is cheaper and much less time consuming than catching them in an act of dishonesty.
A uniformed guard can be very effective in deterring customers and employees in gross acts of dishonesty. For example, a parking lot security officer who makes regular rounds can cause a thief to go elsewhere. An officer checking departing employees at a warehouse has a damping effect on pilferage.
Plain clothes officers are essential for stopping shoplifters. You have to either catch them or force them out of your store. The security officer must be very good at reading people and behaviors in order to differentiate between regular customers and “potential” thieves. Experienced shoplifters, at least the ones who are more cautious, can be deterred by the presence of plainclothes security. Many times security personnel will make their presence obvious in order to scare off the thief. Casually following the person is usually effective.