Definitely this. I have been an Android user since my first smartphone. I've been a Gmail user much longer than that. I still to this day don't have the slightest idea what the distinction between Hangouts, Chat, and Messages (and whatever else I'm forgetting... Wave?) even is. Sometimes my new phone has an app called Chat, sometimes it's Messages, one time it was Hangouts... they all seem to be the same app to me.
So I just use WhatsApp.