This question should be stickied since it comes up nearly every day.
The reason the Black Star is better is because the world is littered with enemy NPC's, all of whom yield the equivalent of a Grand soul. It's much more of a chore to gather grand souls from animals. Mammoths are the only thing that comes to mind that you can reliably locate... but they are much harder to kill than a bandit.
Go with the Black Star every time.

I have the black star now D: Azura iz mad bro..