Earlier quoted context omitted.
Privacy-wise Whatsapp is probably better due to E2E.
How do you know it has E2E? The app itself is closed source. As far as anyone is concerned, it's all claims. How do they handle the private keys, for example? Any backdoor?
The same can be said of Signal or any other chat application distributed through Google Play. How do you know the binary corresponds to the source? Good luck getting reproducible builds on Android or iOS. If you want to be sure your chat app is secure, you need to review and compile the code every time. And, of course, you need the knowledge and skills of a good cryptographer to determine hidden backdoors in the algorithms.
Whatsapp is reasonably secure, as long as you don't upload your unencrypted chats to Google Drive (the backup functionality). Telegram, with E2E enabled, hasn't been proven insecure enough despite its weird custom crypto scheme. However, WhatsApp brings E2E to group chats where Telegram needs manual configuration in private chats to do so. If we want to bring E2E to the masses, WhatsApp is the best option for now.
I'm hoping Matrix will change this or Telegram will implement proper crypto, but until then, WhatsApp is probably the best option we have.