Performance is going to be the same.

Security is the main point here.

If this is your internet facing firewall then you want minimal layers of software complexity, so bare metal is the answer.

I’m a pfSense user, so I don’t know how regularly OPNsense is updated, but, it’s so much easier to just reboot that 1 box whilst everything else is mostly unaffected.

Better still, do a full device backup before an update and then you have a simple disaster recovery backup in case of any problems.