BroadcastReceiver or Messenger via Handler
If your IntentService does not know whether the activity will exist (e.g., might have been destroyed), or if there are multiple activities that might be in the foreground and would want to know about what's going on, I'd use sendOrderedBroadcast(). You can arrange to then also have a "backstop" BroadcastReceiver that could raise a Notification, if desired, as I outline in this blog post and demonstrate in this sample project.
Either of your techniques can work, though.