Signal seems to use less storage space and resources than WhatsApp, it has a very similar user interface which means no learning curve to speak of, and it’s not owned by meta. The last part is very important because unlike Instagram, messenger, and WhatsApp, if Facebook experiences downtime or service interruptions, signal will likely still work fine.
Anyway, there’s a reason why the top political brass use signal as opposed to simplex or Briar, which are even more private. On top of being a very secure and privacy friendly IM platform, Signal just works, it’s very reliable and easy to use, and it has a reputable nonprofit behind it.