Difference between using pathForType and inflector.irregular in ember

We can customize the ember pluralization in two ways;

1. using 'pathForType' in RESTAdapter, like,

App.GroupAdapter = Ember.RESTAdapter.extend({
    pathForType: function(type) {
       var camelized = Ember.String.camelize(type);
       return Ember.String.singularize(camelized);
    }
});

2. By using Ember.inflector.irregular() in model

Ember.inflector.irregular('group', 'group');

In both case it will make a call to 'group' instead of 'groups'. Is there any difference between both? which one to use?