Backbone how to construct json correctly

I want to construct an app of hotel and rooms.
Every hotel can have more rooms, I retrieve this data from external server in XML, I parse it and now I have divided into two arrays: hotel and rooms like this:
hotel.json

[
  {
    "id": "1", 
    "name": "Hotel1"
  }, 
  {
    "id": "2", 
    "name": "Hotel2"
  }, 
  {
    "id": "3", 
    "name": "Hotel3"
  }
]

rooms.json

[
  {
    "id" : "r1",
    "hotel_id" : "1",
    "name" : "Singola",
    "level" : "1"
  },
  {
    "id" : "r1_1",
    "hotel_id" : "1",
    "name" : "Doppia",
    "level" : "2"
  },
  {
    "id" : "r1_3",
    "hotel_id" : "1",
    "name" : "Doppia Uso singol",
    "level" : "1"
  },
  {
    "id" : "r2",
    "hotel_id" : "2",
    "name" : "Singola",
    "level" : "1"
  },
  {
    "id" : "r2_1",
    "hotel_id" : "2",
    "name" : "Tripla",
    "level" : "1"
  }
]

Into my backbone app I have to make some controller and some parse to retrieve rooms for its hotel.
I want to know if is better for backbone to construct a Json like that:

[
      {
        "id": "1", 
        "name": "Hotel1",
        "rooms": [
                 {
                   "id" : "r1",
                   "hotel_id" : "1",
                   "name" : "Singola",
                   "level" : "1"
                 },
                 {
                   "id" : "r1_1",
                   "hotel_id" : "1",
                   "name" : "Doppia",
                   "level" : "2"
                 }
                 ]

      }, 
      {
        "id": "2", 
        "name": "Hotel2",
        "rooms": [
                 {
                   "id" : "r2",
                   "hotel_id" : "2",
                   "name" : "Singola",
                   "level" : "1"
                 },
                 {
                   "id" : "r2_1",
                   "hotel_id" : "1",
                   "name" : "Doppia",
                   "level" : "2"
                 }
                 ]
      }, 
      {
        "id": "3", 
        "name": "Hotel3"
      }
    ]

Which is the better mode for backbone in terms of efficiency and parsing? I thinked the first case but after construct the app I'm not sure.

I would recommend keeping the data structures flat, as Backbone doesn't really support nested collections without some extra effort. Keeping the data model flat will also make it easier for you to map to REST endpoints (ie. '/hotels/1/rooms', 'rooms/1', etc.).

Just to demonstrate the complexities, here is an example of how one would have to associate a collection to a model:

HotelModel = Backbone.Model.extend({
    initialize: function() {
        // because initialize is called after parse
        _.defaults(this, {
            rooms: new RoomCollection
        });
    },
    parse: function(response) {
        if (_.has(response, "rooms")) {
            this.rooms = new RoomCollection(response.rooms, {
                parse: true
            });
            delete response.rooms;
        }
        return response;
    },
    toJSON: function() {
        var json = _.clone(this.attributes);
        json.rooms = this.rooms.toJSON();
        return json;
    }
});

With a flat data structure, you could do something like this:

HotelModel = Backbone.Model.extend({
    idAttribute:'hotel_id',
    urlRoot:'/hotels'
});
RoomModel = Backbone.Model.extend({
    idAttribute:'room_id',
    urlRoot:'/rooms'
});

HotelCollection = Backbone.Collection.extend({
    url: '/hotels',
    model:HotelModel
});
RoomCollection = Backbone.Collection.extend({
    url: '/rooms',
    model:RoomModel,
    getByHotelId: function(hotelId){
        return this.findWhere({hotel_id:hotelId});
    }
});