There are, however, some upcoming developments which will change the situation in the next couple of months:
1) The main matrix.org client, Riot (https://riot.im) has end-to-end encryption now in beta. This will offer Signal-strength encryption, but in a decentralized, e-mail-like system with federated servers. This will create an ecosystem where people are no longer dependent on the goodwill (and solvency) of a single entity to use a good, encrypted messaging app.
2) Briar (https://briarproject.org) is a new (Android-only) app, designed for people with an especially high need for privacy. It works without central servers (through Tor hidden services, but hides the complexity of that), even works when the internet is down (e.g. when mobile networks are shut down during a protest) via Bluetooth and direct Wi-Fi connections, and it offers extra features, like a panic button that deletes all your data. It's in beta at the moment, with a planned release early next year.
TL;DR: Use Signal or Wire for now, but be ready to switch to a better system when available.