WhatsApp uses the Signal protocol but is owned by Meta, which collects metadata even where it can’t read messages. Signal is the original, run by a non-profit, and treats your contact graph as private.
Phone number is still required, so true anonymity is out. The user base is smaller than WhatsApp, so adoption is the friction. Backup story is rougher; if you change phones you can lose history without setup. No multi-account on one device.
Signal is free, funded by the Signal Foundation. WhatsApp is also free; the cost there is metadata flowing into Meta’s ad graph.
Anyone with friends willing to install one more app, especially for sensitive conversations. If you’re the only person in your circle on Signal, the network effect won’t carry the swap.