OpenBSD is more secure
FreeBSD is higher performance and more functionality
Neither is “better” they’re both tools for different jobs IMHO. Which one is “better” for a particular box depends on what tasks you expect that box to do, and what trade-offs you are willing to make.
If you want a hardened internet facing server - maybe consider OpenBSD. If you want a higher performance application server with more functionality (or a desktop), put FreeBSD behind it. Not saying FreeBSD can’t face the internet of course. But that’s what OpenBSD is focused on.
OpenBSD code is heavily audited. FreeBSD relies on upstream for security patches. OpenBSD does not. OpenBSD audit everything themselves and thus they find a lot of bugs that others continue to ship. They do feed back to upstream, but the policy and focus is different. OpenBSD is security over everything else. FreeBSD is not. FreeBSD is more concerned with performance, functionality, etc. SMP for example was way behind on OpenBSD vs. FreeBSD for example (probably still is) because the focus on that was not there with OpenBSD.
Not saying FreeBSD do not care about security, but the level of focus is nowhere near that of OpenBSD. OpenBSD will refuse to ship or include code that they do not feel meets their standards, even if it means that functionality suffers. This means things like driver support, application support, etc. is generally better on FreeBSD than OpenBSD.