Hey, If it helps, i was asking something similar to this, and from what i was told (and from my own opinion from what i have heard from clips):
The Mules: have less output than the BDs, To me they have a sweeter tone and sound brighter and clearer.
The Blackdogs: have more output than the mules, and to me they give a darker kind of sound but they provide better distorted sounds and great bite. kinda a warm sound...
Im not sure if that helps, but it was worth a mention incase u didnt know any of that...
CS