I've used them both - Mark does regular clinics nearby and I called Bruce out. They are slightly different in that Mark is a Chiropractor and Bruce an Osteopath. Both well qualified and very thorough, and I would use either of them again - you just have to decide what it is you really want...
Have to say with my quirky and slightly crooked mare I got more improvement from Bruce than from Mark, but that was just an isloated case and they saw the horse several months apart so may not be a fair comparison.