trust: The biggest trust factor difference to me is, who manages the package and how it is installed. Both are not packaged by my distribution maintainers, therefore I have trust issues with a program that important. However both are available as Flatpak. So I would recommend to install it this way.
updates: Another big factor is how often these are updated, especially security patches. In example for any Firefox based browser, I would not want to wait longer than 1 day before the fork is on the same version as the mainline Firefox.
I personally would prefer LibreWolf over Mullvad, because it is based off Firefox.