WhatsApp has "recently" deployed end-to-end encryption using the Signal protocol, which is of course also being used by Signal itself. The related white paper (PDF).
Now this raises the question:
Is there still any security benefit to use Signal over the much more widely deployed WhatsApp, now that both have good end-to-end encryption?
The threat model in this case includes basically anyone not having access to the phones at the ends. It especially includes the service provider and law enforcement.