That's a very good question and to be totally honest it's not something we've put much research into.
We use ebony pins because we like them .... they look good and seem to work well for sustain and tone although I'm not sure exactly how much effect they have compared to other factors and it could vary from guitar to guitar.
And then again, even if there was a difference it might not be a difference to make them "better" ... just different.
Some players like ebony, some plastic or brass .... some even splash out for fossilized walrus ivory
each to his own I guess
For me it's ebony .... looks good and works but I also like Black Buffalo horn to ... oops .... maybe I'll say no more