I work in the alarm industry, all I can say is do your research, take your time, and don't rush to a decision.
Ask as many questions as you can think of when speaking with wich ever alarm company, and make sure you read any contracts they have you sign.
Many companies may offer a free system, or a basic package at a discounted rate, but may not have the right coverage for your home, so make sure you ask exactly what kind of coverage to expect.
And ask what exactly you recieve for their monthly fee. I have had customers that switched to the company I work for, who paid $10/month more with their previous company, but didn't recieve anything extra, basically they were paying more for the same level of service. But some companies may charge a bit more, but also offer a bit more (like a service plan built in, whereas maybe another company requires you to pay extra for a service plan)