If you want a large company that has many options for your home security, and a large monitoring center then you choose ADT. Starts at $27.99/month:$32.99 with smoke or heat detectors. They are trained to sell up when giving you a system.
If you want a medium sized National company and good monitoring and lower monthly cost, go with Brink's $26.99/month: $28.99/month with smokes or heats. They are trained to sell to you in the $750-$1,000 price range for install.
If you want a company which offers a diverse amount of products, meaning brands, then go with a local. However be careful. Local companies don't have as much experience usually, and many go out of business fast, and then you are screwed. Do you homework. Better business bureau, small business administration, and the chamber of commerce. These companies, can sell monitoring as low as $20.00/month. The installs are usually more expensive.
Recap: Basic system with Brink's $199 $26.99/month
Basic System with ADT $372 $27.99/month
Basic System with local $500 $20.99/month
Beware of the ADT Authorized dealer. They greatly overcharge the monthly monitoring, and are not really related in any way to ADT, this is just a way for ADT to get more monitoring accounts. Good luck. I have worked in the security industry as an expert consultant for 10 years. I hope this helped.