Meta sees your entire social graph by design, and stores the name and participants of groups you create. You (meaning: experts) can independently audit and through reproducible builds verify that Signal does not do this by design, while nothing about WhatsApp can be audited in the same way. Meta is also a plainly for-profit company with bad track record, while Signal often gets flak from (IMO mistaken) radical nerds but otherwise has a very good reputation.

The privacy fatalism in your comment here and in other comments is plainly incorrect.