While it isn't obvious at first glance, Signal is definitely more secure than WhatsApp. Both products use secure end-to-end encryption for the content of their messages. They are both equally secure as far as that goes. In fact, WhatsApp's end-to-end encryption uses the Signal protocol to encrypts its data too. But Signal encrypts your metadata, while WhatsApp logs as much of your metadata as possible, and passes it along to Facebook. While metadata doesn't expose the contents of your messages, it does include information on who you talked to, when you connected, and much more.