Lets start with their origin. Bullmastiffs are probably one of the oldest breeds in the world and they were bred to be great guards and hunters. Its a dog from United Kingdom and it is believed, that they have Great Danes, St. Bernards and Bloodhounds in their lineage.
The important difference is in their mask. Bullmastiff always has a black mask and black nose. At least all purebred Bullmastiffs must have it. It is true, that some Boerboels also have black mask, but it is not true for all of them. There are a lot of Boerboels, that have color of their face same with color of their coat.
The size is very similar for both breeds, around 65 cms (26 inch). But Boerboels are definitely heavier breed. They weigh between 60 to 80 kgs (140 to 180 pounds). Bullmastiff is lighter, but its still very big dog with weight between 50 to 60 kgs (110 to 140 pounds).
Both breeds are pretty dominant and i would not recommend them to novice dog owner. Both breeds are also excellent family dogs and they can live with kids with no problem. But you should socialize them properly, just like any other breed. Both are also highly intelligent, which makes them highly trainable. But its important to start the training as soon as possible, because both breeds are pretty independent and can be stubborn sometimes.
Watch our full comparsion between Boerboel and Bullmastiff: