I assume that you are referring to Malinois when you say BSDs, but my comments apply equally to Groendals and Tervurens...
It is sort of hard to generalize across breeds and bloodlines inside breeds, but here is my experience: most Mals and their relatives are very handler focused and sensitive. This might seem to make them somewhat easier to train, but that comes at the cost of being less resilient to trainer mistakes. You can ruin one of these dogs much easier that you can most GSDs. That said, however, certain bloodlines in GSDs (older East German and some high or show lines) are just as handler focused and sensitive, and can have similar training results and issues.
As for being biddable, I don’t think either breed has a lock on this, as there are bloodlines in each breed that contain this trait. Think of it as a personality trait in people, and you will understand what I am getting at... And the independent watch dog trait you mentioned is actually more related to territorial guarding, and is present in almost all dogs, particular those bred for herding and work. So, this will be present in varying degrees in both breeds.
Bottom line: if you are looking for a certain type of dog, look at the breed, look at different bloodlines inside the breed, and then talk with trainiers who have similar training techniques as you plan on using, and get a dog that is compatible with you and your training methods.
Excellent answer. Thank you very much and I find dog breeds to be very interesting.
I am an American Brittany man myself and get my dogs from Beeline Brittany. (I know you didn’t ask but I love these dogs so much I take very opportunity to say something.)
Just to add to what you said, my first brit was the last dog taken from the litter and his breeder said he would not make a good companion dog as he was always going off by himself. After working with him for a few years, what I found was that he just had a much longer range and had fantastic drive (more german pointer than a german pointer). He was all work. He would even nip at other dogs if they distracted him while he was working. Once the sun went down (no birds to look at), he was a great companion. This breeder is the top guy in the breed and even he didn’t see it. I was lucky. His grandfather was an all breed champion and I’m convinced that mine would have been too if he had a better owner. (Odd to feel unworthy of a dog, isn’t it?)