Collection: Array vs Object?

ArrayCollection.js

/**
 * ArrayCollection
 */
function ArrayCollection()
{
    this.ids   = [];
    this.items = [];
}

/**
 * Clear
 */
ArrayCollection.prototype.clear = function()
{
    this.ids.length   = 0;
    this.items.length = 0;
};

/**
 * Count the size of the collection
 *
 * @return {Number}
 */
ArrayCollection.prototype.count = function()
{
    return this.ids.length;
};

/**
 * Add an element
 *
 * @param {Object} element
 *
 * @return {Boolean}
 */
ArrayCollection.prototype.add = function(element)
{
    if (this.ids.indexOf(element.id) < 0) {
        this.ids.push(element.id);
        this.items[this.ids.indexOf(element.id)] = element;

        return true;
    }

    return false;
};

/**
 * Remove an element
 *
 * @param {Object} element
 *
 * @return {Boolean}
 */
ArrayCollection.prototype.remove = function(element)
{
    var index = this.ids.indexOf(element.id);

    if (index >= 0) {
        this.items.splice(index, 1);
        this.ids.splice(index, 1);

        return true;
    }

    return false;
};

/**
 * Does element exists in collection?
 * 
 * @return {Boolean}
 */
ArrayCollection.prototype.exists = function(element)
{
    return this.ids.indexOf(element.id) >= 0;
}

/**
 * Is the collection empty?
 *
 * @return {Boolean}
 */
ArrayCollection.prototype.isEmpty = function()
{
    return this.ids.length === 0;
};

/**
 * Get the index for the given element
 *
 * @param {mixed} element
 *
 * @return {Number}
 */
ArrayCollection.prototype.getIndex = function(element)
{
    return this.ids.indexOf(element.id);
};

ObjectCollection.js

/**
 * ObjectCollection
 */
function ObjectCollection()
{
    this.items = {};
}

/**
 * Clear
 */
ObjectCollection.prototype.clear = function()
{
    for (var id in this.items) {
        if (this.items.hasOwnProperty(id)) {
            delete this.items[id];
        }
    }
};

/**
 * Count the size of the collection
 *
 * @return {Number}
 */
ObjectCollection.prototype.count = function()
{
    return Object.keys(this.items).length;
};

/**
 * Add an element
 *
 * @param {mixed} element
 *
 * @return {Boolean}
 */
ObjectCollection.prototype.add = function(element)
{
    if (!this.items.hasOwnProperty(element.id)) {
        this.items[element.id] = element;

        return true;
    }

    return false;
};

/**
 * Remove an element
 *
 * @param {mixed} element
 *
 * @return {Boolean}
 */
ObjectCollection.prototype.remove = function(element)
{
    if (this.items.hasOwnProperty(element.id)) {
        delete this.items[element.id];

        return true;
    }

    return false;
};

/**
 * Does element exists in collection?
 * 
 * @return {Boolean}
 */
ObjectCollection.prototype.exists = function(element)
{
    return this.items.hasOwnProperty(element.id);
}

/**
 * Is the collection empty?
 *
 * @return {Boolean}
 */
ObjectCollection.prototype.isEmpty = function()
{
    return Object.keys(this.items).length === 0;
};

/**
 * Get the index for the given element
 *
 * @param {mixed} element
 *
 * @return {Number}
 */
ObjectCollection.prototype.getIndex = function(element)
{
    return element.id;
};