Just looked up the difference (because I don't use Facebook I wouldn't know).
*WhatsApp prioritizes privacy (with default end-to-end encryption) and is built around phone numbers for straightforward, direct communication. It's often seen as a more private and secure alternative to traditional SMS.
* Facebook Messenger is more integrated into the broader Facebook social network, offering a wide array of social features, games, and integrations, but with less emphasis on default end-to-end encryption.
The choice between them often comes down to individual priorities: privacy and simplicity (WhatsApp) versus a richer games and feature set and full social network integration (Facebook Messenger).
I suppose if you just want a call, message, video and image mechanism then WhatsApp may be better but if you want a social network integration where you can see everything about someone and be more fully involved the Messenger system may suit you.
I think WhatsApp suits me as I can select exactly what is shared with who...
No idea how this works with Messenger.
If you have someone in your Messenger chat thingy can they see all your photos for example?