These two messengers have their own benefits based on what you are planning to achieve with them.
While the open-source technology and sophisticated security of Signal make it a secure alternative, it's not designed for group communication.
With lots of functions and the Threema Work application, Threema advances team discussions a step forward. But it still isn't developed in this industry.
It's important to note that Signal is a free app, while the Threema app requires payment of a certain amount of money.