IMO you want the right tool for the right job. Getting one tool for 2 such vastly different jobs will lead to a crap family pet, and a crap guard dog. To me, the idea of guard dogs are stupid in itself. They're not going to stop anyone determined, probably just end up with a dead pet. And what happens when the little boy next door jumps the fence to get his ball back?
Just get good home and contents insurance. It's cheaper and safer, and if you get broken into they replace all your shit with brand spanking new stuff. Sweet as.
A good family pet has never experienced violence and aggression.
Most of the mastiff and mountain dog breeds wouldn't do shit to an intruder, but their looks alone can be a deterrent in itself. But you should be an experienced dog handler/trainer and raise them right or you can be in for all types of problems.