
As far as Im aware both are quite similair in that they can be boisterous and need a firm hand but are extremely lovable and not all are dog agressive if bred for temperament and well socialised when a puppy. I have staffys but love the bull terrier too- although I have never owned one (yet).
If you opt for the bully I would check that all health tests have been done prior ro the breeding as they can be prone to Kidney disease. Im sure if you contact one of the bully clubs they would be more than happy to talk about the breed :)
Good luck - whichever breed you choose :)