The best dog we ever had was a German Sheppard/Chow-Chow/Mutt mix. He was loving, protective, and lived to be about 14.
As far as having a dog to guard the family nothing beat the Akita we had. However she was not good around non-family members at all. We'd leave the room and she would corner company we had over.
Jen, my partner, works at a vet clinic and sees all breeds of dogs. She recommends mutts to family and friends because of longer lifespans and fewer medical issues. So my personal opinion is that you find the size and temperament you want in a dog and shop the pound for animals that fit.