The decision to hire either a uniform or a plainclothes guard should be based on whether it is more important to deter (potential) thieves or to catch them. Keeping people honest is cheaper and much less time consuming than catching them in an act of dishonesty. Overall, deterrance is preferable. If you catch an employee stealing and fire them, it will cost you to hire and train a replacement. If you decide to have them prosecuted, you will spend many hours not making any money.
A uniformed guard can be very effective in deterring customers and employees in gross acts of dishonesty. For example, a parking lot guard who makes regular rounds and investigates car alarms can cause a thief to go elsewhere. A guard checking departing employees at a warehouse has a damping effect on pilferage.
Plain clothes guards, on the other hand, are essential for stopping shoplifters. You have to either catch them in the act 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 those who steal. Experienced shoplifters can be deterred by the presence of plainclothes security because they know when they are being watched. Many times security people will make their presence obvious in order to scare off the thief. Casually following the person is usually effective. These type of guards tend to be more physical since their job also includes catching, subduing, and arresting the thief once he or she has left the store.