Both messengers come from the same stable (technology-wise) but how they implement it and what the defaults, is what separates them.
See http://lifehacker.com/secure-messaging-app-showdown-whatsapp-vs-signal-1794684943 
Secure Messaging App Showdown: WhatsApp vs. Signal
So, you’re interested in secure, encrypted chat apps. You have a few different choices, but as with any chat app, what all your friends are using is important. To that end, Signal and WhatsApp are easily the most popular. Here’s how they compare.