Black one can only hold human souls, which are always grand sized. The other star can hold any soul, and if it's empty and you don't have any empty soul gems and kill something with a lower than grand soul, it will go into the star.
So, the black star will ALWAYS have a grand soul in it. With the other it's possible that it will be smaller.
I got the black star, and keep a bunch of lower level soul gems in my inventory for smaller souls to go into so they aren't wasted.
THE BLACK STAR WILL CAN AND WILL ABSORB WEAKER SOULS!!!
it's evidently not supposed to, but it absolutely does.
it is still the better choice because it CAN absorb human souls, you just have to make sure that the first thing you absorb once it's empty is a human soul. but you don't even have this option with the regular star.