How biddable are BSD’s compared to GSD’s? I was a bit surprised to read BSD’s are easier to train but perhaps GSD’s have more of an independent watch dog trait in them?
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.