Rails: Model instance method or helper method?

By convention, should the following be defined as an instance method of my model or a helper method?

# app/models/user.rb
class User < ActiveRecord::Base
  def full_name
    "#{first_name} #{last_name}"
  end
end

or

# app/helpers/users_helper.rb
module UsersHelper
  def full_name
    "#{@user.first_name} #{@user.last_name}"
  end
end

Many thanks.