Second, Signal only stores account creation date and I believe account deletion, or some other super-basic info like that. But that's it. Meanwhile, not only does Whatsapp collect much more metadata, including the groups of which you're part, but it has also started sharing that data with Facebook recently.

I'm not saying you should stop using Whatsapp, though. I still think it's a better alternative to iMessage/Hangouts/Skype/Telegram. But at the same time you should slowly transition your most important contacts to Signal, too. One nice extra feature Signal has over Whatsapp now is Snapchat-like disappearing messages, so at least you may be able to convince some friends with that.