One thing off the top that dnsmasq can do that unbound can not is do localized responses.. Not aware that unbound can do that? Pretty sure dnsmasq will send queries to all dns servers listed and use the fasted response. I believe the way unbound does it is sequential?
As cmb states there are differences in for sure.. dnsmasq is by design a forwarder, while out of the box unbound is meant to be a resolver while it can be put in forwarder mode that is not where it shines so having both available for sure makes better choices for pfsense. Now if they had an authoritative dns that would be the homerun like bind..