I was using Messenger on my Note 4, so being used to it, I continue to use it. It's simple, the MMS UI is great - love the in-message camera, and I'm accustomed to it. I attempted to use Hangouts about a year ago, but just didn't like it. I don't send or receive any Google messages, so it seemed a bit pointless. It'll be Messenger for the foreseeable future.