They are very different platforms, Signal offers on-device encryption if you choose to enable it, prompting for a password when starting Signal after rebooting the phone. It will also keep a quick lock option in the menu on Android phones so you can easily prevent unauthorized access.

I believe Whatsapp has made a few compromises in this regard, but obviously Michael Cohen didn't bother to use disappearing messages in Signal or encrypt his Signal DB, despite how easy it is to do.