Security systems are not all created equal, just because they are expensive or inexpensive doesn't mean they will work for your home. 

Always ask a lot of questions like:

1. Am I leasing or will I own this system?

2. Will my agreement be sold to another company?

3. Whole will service my alarm system and can I have their name and number to check them out?

4.Can I break the agreement without a penalty, say that I sell the house or move?

5. Glass breaks are always better then motion detectors, because in order for a motion to go off, the person needs to be inside the house.  For a glass break to be operational, the person will be on the outside of the house.

6. You and your family should always have a plan should there be a break -in.