One itty bitty caveat.
As I understand it, layoffs of unionized workers occur by strict seniority: the youngest, most recently hired workers go first. (This is certainly true in Φ's flyover country.) So union members, facing either an X% cut in benefits or X% layoff, know exactly whether or not they will get to keep their jobs if the union votes for the layoffs. In the private sector, the members are – or should be – vaguely aware that their firm is subject to market discipline and that the company will go bankrupt if it can’t make its union contract profitable. But this isn’t true for the public sector; the schools, alas, will always be with us.
It follows that if X < 50% and if each member votes his individual financial interest, a majority of the union will always choose the layoffs over the benefit cuts because that majority will still have their jobs after the layoffs. So Christie’s criticism isn’t entirely fair since the teachers laid off almost certainly voted for the benefit cuts.