Mediation was traditionally voluntary but now has been (poorly) incorporated as part of the court process in Ontario. Mediation is an attempt to resolve the matters in dispute with the assistance of a third party by way of discussion and compromise.
Good Matrimonial lawyers routinely try to resolve issues in this fashion. They may have a tendency however, to resort to the traditional adversarial approach when mediation hits a roadblock. Some issues are more difficult to arbitrate than others. Custody is often such an issue.
Arbitration does not focus on what each party is entitled to, so much as what works for the parties and seeks common ground on all of the issues. Experienced matrimonial lawyers prefer a mediator who is a matrimonial lawyer or a retired Justice with a matrimonial background. These people are grounded in what each party is entitled to and is given up as part of the resolution.