Each does something entirely different. An alarm system will help to protect the items that aren't usually kept in the safe. It will also reduce the amount of time that a bad guy has to work on the safe.
However, this does not change the fact that you should be using the proper safe for the job. Back before alarm systems, banks and jewelry stores would place their safes within plain view of the windows. Not only did this discourage bad guys by making them visable to those passing by, but it also discouraged them by letting them know that they wouldn't have an easy task facing very formidable security.