If you do go the route with monthly monitoring, make sure to negotiate lower contract commitment. Many companies ask for 3-5 years, play them off one another and try to get 1-2 year contract (say the other company is offering a shorter contract).
Not many people realize how easy it is for the thief to inactivate the home security system. All they need to do is to cut the phone or cable line (depending on which you have home phone service for). The alarm will still sound, but it will not be able to contact the security company or police.
Only hope would be a neighbor to call for you in a situation like that.
I would HIGHLY recommend getting a wireless cell back up (it does cost more monthly for monitoring) but it will still allow for the system to contact the security company and/or police if the phone line is cut. Have it hidden up in an attic to make it difficult to get to.