[Open]ACS 4 has a more general model, acs-relationships, that map objects+roles to other objects+roles in a one-to-one, one-to-many, or many-to-many relationship. Support for groups and the like are built upon this.
So [Open]ACS 4 does it much more generally. Whether this is better or not is probably a matter of opinion but it's certainly a lot more flexible.
