WhatsApp has a userbase in the millions, if not billions. It may use the same protocol as Signal (I wasn't aware it does), but its existence is marketed as and

WhatsApp has a userbase in the millions, if not billions. It may use the same protocol as Signal (I wasn't aware it does), but its existence is marketed as and understood as that of a messaging app. Signal's identity, in contrast, is predicated upon security. Based on that, along with the fact that Whatsapp is owned by Facebook and so its functionality can be changed by its owners, I would also probably advise acquaintances concerned about privacy to use Signal over Whatsapp. (Personally, I've been using Threema for E2E encrypted communications with friends who care about privacy enough to not want to use Facebook Messenger or other Facebook-owned platforms.)