A pooch can certainly add another level of protection to your home, whether you want one in addition to or in lieu of a security system.
But not all dogs are equal when it comes to keeping your property and family safe. Guard dogs protect and defend property. They're not pets. Protection dogs, on the other hand, are family dogs that watch over people rather than a physical area. German shepherds, rottweilers and Doberman pinschers are among the best breeds for home protection, thanks to generations of breeding. They respond well to discipline and are friendly with their families, including children.
If you want a security-trained dog, make sure to buy one from a reputable trainer. And if you want a dog and security system, look into pet-immune motion detectors that will announce intruders rather than your furry friend!