Graphite
Matter of convinience, minimalism + reasonable trust in Session, and the fact that files are few and not of great importance.
It would definitely not be a good way to transfer gigabytes worth of photos. But then again, I wouldn't use local network for that either, just a USB drive.
That said, if one had to use a messenger, Briar would probably be a better option, exactly since it's decentralized. But none of my contacts use it, so no experience there.