There is no way that anyone can guarantee that you will never be robbed. A security system is a deterrent and in most cases will provide you with additional protection over other methods.
You must look at the additional costs of having a security system installed and monthly monitoring costs compared to the increased protection. People that have alarm systems still have their homes broken into. They are 3 to 4 times less likely to have this happen than homeowners without an alarm system. Those are pretty good odds to have in your favor.