Big obvious problem with WhatsApp is that it's owned by Facebook and seeing as WhatsApp is free they're most likely using it as a data collection / tracking mechanism. Signal is much more based around privacy, it's open source and Open Whisper Systems are funded more transparently through grants and donations.
But ignoring that and just looking at the features.
Typically I use them for different things. WhatsApp works better for sharing with groups of people because lots of people have it on their phone.
I use Signal as a layer on top of SMS but typically only 1-1 communication, so if both of us have Signal then great, but otherwise it's just regular SMS.
While the messages themselves are definitely E2E encrypted using Whisper Systems' specs (which is nice), the problem is that once the app decrypts them, I have no way of verifying that Facebook does not steal my message for profile building purposes.
Signal is easier to use. I use it primarily as a SMS client but when I'm talking to someone who is also using Signal the conversation automatically becomes way more secure.
No one I knew used WhatsApp or signal (I’m in the US). However I just finished a project with a customer spread between Middle East, Singapore, and India. They ALL used WhatsApp and asked us to use it to communicate with them.
So definitely seems to vary in popularity depending where you are located.
I quit all social media and that included WhatsApp, about a year and a half back. Everyone is fine with me leaving social sites but how dare I leave WhatsApp? They look at me weird for that.
Surprisingly though, Signal is slowly gaining popularity among the smarter crowds.
But ignoring that and just looking at the features.
Typically I use them for different things. WhatsApp works better for sharing with groups of people because lots of people have it on their phone.
I use Signal as a layer on top of SMS but typically only 1-1 communication, so if both of us have Signal then great, but otherwise it's just regular SMS.