Depends. I have Belgian Shepherd Dogs (Tervueren and Groenendael)...and it really depends on what you want from a personal protection dog.
There are lines of Tervueren and Groenendael that suffer from weak temperament, so if you are looking for a Tervueren, as a personal protection dog to train professionally, as in high level Schutzhund, French Ring or IPO-training, you will have to look very carefully, as there are very few breeders in the world that consistently produce Tervueren in that quality. In this case a Malinois from a good breeder would be a far, far better choice.
If you are merely looking for a dog that will protect you, then a well-bred Tervueren (again be critical about temperament when you look for the right dog) will most likely do this instinctively, as they have been bred not only to herd, but to protect livestock. My Tervueren certainly did. My Groenendael is still young and a bit softer, so while she guards the property with vigor, I can't say if she would protect me or not. Not that it is her job anyway.
Whether you should look at a BSD or a GSD...it will depend on you, your personality and how good a dog trainer you are. The Belgians are more difficult to handle and less forgiving toward handler mistakes than the Germans. I know of experienced GSD handlers who got their first Malinois and who were completely taken by surprise at how different it was to train a Mal compared to a GSD.
Training a breed typical Belgian is like walking a fine line, be too hard on it and it will not cooperate, be too soft on it and it will walk all over you, whereas the GSD will give you a bit more leeway. If you don't manage to find that right balance with your BSD, it can be every bit as stubborn as a Rottweiler. So be realistic about your own dog training experience and capabilities, and do read up on various training methods, so that you are prepared.
Both BSD and GSD are intensive and handler sensitive, but the Belgians are exaggeratedly so. If you get that symbiotic relationship with your BSD - then is it something for you? There is a saying that either it's "one time BSD- always BSD or one time BSD- never BSD again". I know of military dog handlers who have gone either way: from GSD to BSD and never looking back, but I also know of some who tried the BSDs and went straight back to the GSDs, because of the intensity and owner fixation of the BSD.
BSDs also need to be activated more mentally and physically than the GSDs, so you need to take your daily schedule into consideration. These are dogs that are right behind Border Collies when it comes to activity levels.
The BSDs have less physical health problems as a breed than the GSDs, although you still need to keep an eye out for hip-dysplasia. Tervueren and Groenendael, especially Groenendael, have some problems with epilepsy, but epilepsy is one of the illnesses that is relatively easy to weed out of breeding lines, so if you get your dog from a good breeder that checks for this, then it shouldn't be a problem.
- The working line GSDs tend to be more vocal than the BSDs.
- GSDs mature faster mentally than BSDs. With GSDs you will often have a reasonably mature, sensible dog at 12-18 months old. With a BSD, you shouldn't expect to have one until it's 24-36 months old.
- BSDs are naturally dog-dominant and will have a high incidence of this in the breed. More so than the GSDs.
- With BSDs the herding instinct is closer to the surface than with GSDs, so if you get a puppy, expect to deal with both puppy mouthing and herding nips.
- For Tervueren, you need to be more careful about checking the dogs temperament when chosing a dog than with GSDs. It will be significantly more difficult to find a Tervueren for high level protection then finding a GSD.
- You can get amazing training results with the BSDs, but it all depends on your handling skills.
- It is easier to train a GSD than a BSD.
Long time BSD owner. Have military dog trainers as friends and have trained my current Groenendael with one, who currently works with working line GSDs. My first Tervueren was partially trained at the local police dog club.