I am a bulldog man. The only thing I'd add to the comment from chris is that bulldogs only tend to defend a home if the low life enters the property from an unusual place. Generally if they come through the front door the bully will, we'll be a bully and pin him down for a cuddle and good lick!
A few years ago now I had an American bulldog of the Alahapa blue blood variety. They have loads of American history and as guard dogs at that. They are extremely trainable. My girls mum was a trained shutzhund and fantastic pet. Her owners also bread thoroughbred stallions who's stables were a ten min walk across they're property, which was the reason for the dogs training. On one command she went from a loving pet to a murderous beast! And back just as quick.
My bitch, although did not receive the professional training, was extremely obedient, intelligent enough to accept her place in my home (she took command not only from me but my wife and two kids) and above all protective. She did not however show any protectiveness without command, apart from wen the doorbell went.
These are great dogs, even crossed with Rottweilers which I know is popular in the states. The call them bullweillers, it just makes the huge!
My brother keeps dobermans and I've noticed they have trouble serving two purposes. They are either great pets or great guards and not a lot in between.
I've not had much experience with German shepards but they use them for police dogs here in the uk which says alot. They l however suffer from blindness, deafness and bad hips. All hereditary so good research and honest breeders can eliminate this.
Being a dog lover to the max I would like to pass on to you what I consider my responsible buying rule. I won't buy a dog who's breeding programme is not one that is breeding out hereditary problems. For example my bully's came from stock with long snouts, long lives, no heart problems or hip displacements or cherry eye. As a breeder it's my responsibly to breed out these ailments. If all breeders did this we could all have healthy dogs.
Good luck and enjoy whatever dog you buy
