Diferença visual entre callbacks e promises

gistfile1.js

// Callbacks
ObjController.prototype.update = function (req, res) {
  model.save(function (err, object) {
    if(err) req.render500(err)

    otherModel.update(req.form, function (err, object) {
       if(err) req.render500(err)
       req.render('obj')
    })
  })
}

// Promises
ObjController.prototype.update = function (req, res) {
  model.save()
  .then(function (object) {
     return otherModel.update(req.form)
  })
  .then(function () {
     return req.render('obj')
  })
  .fail(function (err) {
    return req.render500(err)
  })
}