The Black Ghost is by far the most outgoing of the two.
Brown Ghosts tend to remain shy where Black Ghosts can become tame enough to take live earthworms(one of their favorite foods) from your fingers.
Both do best when kept with only one per tank. In a very large tank more than one may work. That is something I have yet to try. Of the two I suspect Brown Ghosts may be the easier of the two to keep in groups but that is just my hunch.
It would be nice to know more about how they breed. I know Black Ghosts are being bred in SE Asian fish hatcheries but they breed many species by using artificial techniques like hormone injection, egg and milt stripping. I haven't come across any accounts as to how they are producing their Black Ghosts.
Both are among the most interesting of the "commonly kept" tropical fish.