When would you choose dbus-broker over the reference dbus-daemon?

dbus-broker is an alternative, from-scratch implementation of the D-Bus bus daemon, built for performance and tight systemd integration, and it's now the default on many mainstream distributions in place of the historical reference dbus-daemon.

You'd favor it whenever bus performance under heavy message load matters, since it's designed around efficient kernel-level primitives for broadcasting signals and minimizes userspace overhead compared to the reference implementation's more general-purpose routing code. It also integrates directly with systemd's socket and service activation model, which can make service activation and sandboxing more consistent on systemd-based systems.

The trade-off is that dbus-broker deliberately doesn't reimplement every obscure corner of the reference implementation's behavior, focusing instead on the well-defined D-Bus specification, so an environment relying on a very specific reference-implementation quirk or non-systemd init could have reasons to stick with dbus-daemon instead.