Some important points to consider while comparison shopping:

Beware of "free" installations
       
"Something for nothing" gimmicks are very common and often include sparse alarm systems which won't properly protect your premises.    These also generally require monitoring contracts of 3 or 5 years at inflated monthly fees.

Compare the monthly fees
          Make sure you are comparing fairly -- the extra monthly amount can add up over the length of the contract.

Check the credentials
         
Security companies should be licensed by the state in which they operate, and sometimes individual counties require licensing as well.     Also make sure the company is adequately insured and that the technicians have passed certification requirements.    Also see that the monitoring station is UL-Listed.    A listing with Underwriters Laboratories is the best assurance that the monitoring station meets very strict criteria, and are subject to  unannounced inspections.

Organization affiliations
            Check on the company's affiliations with
associations such as the National Burglar and Fire Alarm Association (NBFAA) and the Maryland Burglar and Fire Alarm Association (MBFAA).