Actually my money is still on the tang even if the fish are the same size (in my case the tang was larger). The dorsal fins of the butterfly are no defense against a torqued-off tang. Butterflies are really quite timid creatures. In fact I wonder if slightly aggressive tankmates (ie., where the aggression may not be so obvious to us as casual observers) this contributes to the common story that butterflies starve in our tanks.
Anyhow, my suggestion is still basically ensure that the butterfly has escape routes (creative rockwork) and ensure the tang is well fed, and hopefully within a couple weeks they will be tolerant of one another. My tang and CBB get along fine nowadays (18 months together in the same tank) although my tang will still charge at the CBB if he feels he's not respecting the totem pole (eg. "you're too close to my nori stick, GET OUT OF MY CORNER!!").