Complicated answer... it depends.
Heh, it really does depend on a lot of factors- what you are planning to use the brush for product wise, skin texture, application technique, desired end appearance, etc. the list goes on and on.
Personally, some things I find sable better for, others badger, still others kevrin, or goat, or squirrel, or rabbit, or synthetics. I really don't feel there is a definite X is better than Y answer for this one.