Depends in what context I meet them. If they know my real name or identity, I would give them my phone number so they can add me on WhatsApp. My phone number is connected to my real identity anyway.
But if I am meeting someone in a pseudonymous context, I would give them my Matrix handle instead, so they can add me there. My Matrix account is not tied to my real identity in any sense.
The idea is that someone who knows my real identity should never learn about my pseudonymous identity and activity, and someone who knows my pseudonymous identity should never know my real identity or activity. To keep things separate. At least until I have developed a high level of trust to the person.