Guards vs Cameras
In order to understand the difference between cameras and guards, we must first ask ourselves the question "why do we need security cameras?". Security cameras are a great way of deterring crime and a very good way of providing evidence of the who/what/where/when/how. This technology - Closed Circuit Television - has been in use since 1942. Since then, countless crimes have been solved, thanks to CCTV camera footage. The purpose of CCTV not only provides evidence and a deterrent, but also provides a view covering all desired angles which may otherwise be physically impossible to monitor simultaneously. Typically, this type of surveillance is used together with physical security.
Security cameras are essential in a day and age of technology and electronics dominating the workplace and the general public. Without cameras, many businesses would suffer great losses to their annual profits due to things like thefts or damages occurring.
Cameras Only
Having cameras alone, is insufficient to prevent or intercept a crime-in-progress. Even cameras placed as a deterrent will not stop a motivated criminal from committing an offense. The main problem with having cameras is they can only reveal what happened after the fact. Evenso, this is not guaranteed to provide substantial evidence. If the perpetrator isn't caught at the appropriate angle and is covered in such a manner you cannot see his/her face, then the video footage is essentially useless in discovering "whom" committed the crime.
Even having an individual camera for every possible significant angle, does not stop or prevent the crime from occurring. Having clear of evidence of whom committed any such acts, can be very helpful in pursuing them. However, this does not stop them from committing the act in the first place.
Guards Only
The concept of security guards (physical security) has been around since ancient times. During the days of the Romans and earlier. This method of deterring, preventing, intercepting, and solving crimes; is unanimously one of the most effective means, commonly used in any aspect of life and safety.
With a guard conducting foot or mobile patrols, on the inside and outside of a particular building / perimeter; you are able to see a lot more of what is happening in the environment, as well as increasing the chances of catching someone in the middle of the act. The larger the property, the more guards should be on duty at any given time. This however, is in contrast to the crimerate of your local community - and - in contrast to the number of emergency services available.
Let's say for example, you have a scenario wherein: a guard intercepts an individual comitting an offense on your property, yet there are only two other people there to witness it. The two witnesses join the perpetrator and claim the guard was "abusing his power" and "wrongfully detained" the individual in question.
This can become problematic when you have the validity of a single bonded security guard, with a clean record, who's always doing his reports and hasn't yet falsified a report. Versus 3 other witnesses - eventhough the perpetrator may not be credible - this creates an unusual difficulty in making a convincing case in a court of law, or even simply having to now chastise this likely to be innocent guard.
In some places, the problem of illegitimate witnesses can be overruled having multiple (at least 2) guards respond to any incident, or to do so in the public eye - so as not to be as liable to false accusations.
Ultimately, the purpose of having physical security on any given property, is to keep safe, all assets, property, and lives. Although certain complications come with this choice of security, as previously mentioned. This remains to be one of the most effective means of security.
Cameras & guards
To wrap things up, having cameras or guards alone has their own advantages and disadvantages. Combining the two, would greatly increase the advantages and decrease the disadvantages. No system is perfect, no matter how many different aspects you have covered. However, the more "holes you can patch" the better your overall results will be.
A combination of CCTV cameras, security guards, alarms and sensors, and having someone monitor and supervise the system 24/7. Is what makes security far more effective at preventing or stopping a crime - or any unwanted activity period.
Cameras would provide the viewing coverage, all the angles, and live feed of the environment. Guards would provide additional deterrence along with the ability to intercept / mitigate problems. Having alarms and sensors make it easier for the people monitoring this ongoing security coverage, to be notified of certain things and to more swiftly respond to it. It goes without saying, that all this is bound together by radiocommunications.
Not every scenario, requires such an elaborate system in place. Sometimes cameras are enough, and sometimes physical presence is enough. The point is, that whether it's a small house with 4 cameras, or a massive complex with all the bells-and-whistles; layered security features go a long way for protection and make the whole process worth the investment.