I have used both, and I prefer OpenBSD. There always seemed to be little problems with FreeBSD that I have not found in OpenBSD (I also know a lot more so that could account for a majority of it). One of the people that got me hooked on OpenBSD is actually a developer now (used to use FreeBSD and he convinced me to use that instead of linux). Anyhow, FreeBSD has more ports, but OpenBSD has the major ones. I rarely find a piece of software I want to use that is not ported to OpenBSD. But if I do most of the time it either compiles or there is a work-a-like in the ports. Also OpenBSD can run BOTH linux and FreeBSD binaries. Well a good deal of them anyhow. OpenBSD has a cleaner install, there is not as much "junk", but there is also not as many cool options. I have to wait till the system is up and running before I can install packages in OpenBSD whereas I can install them during the FreeBSD install. But I actually like that.
The OpenBSD developers are gods. They have not only built a rock solid Operating System, but they have also brought awesome software like OpenSSH to the masses, and have now created their own packet filter, aptly named Packet Filter. Also, they go through EVERY FRIGGIN LINE of the code in the operating system so often looking for bugs. They find security holes and silly little bugs BEFORE They become a problem in many cases. I think they fixed around 800 possible security holes in one release... While FreeBSD is good, they have been lacking in the security dept lately and that worries me.
OpenBSD has almost no SMP capabilities right now, although that is being worked on. But this is considered unnecessary by Theo DeRaadt (the asshole behind OpenBSD). So this work is going slowly.
OpenBSD also does a lot of work with encryption, and this is why many (if not most) live outside of the USA. Due to the US' archaic and oppressive encryption export laws, OpenBSD HAS to be developed outside of the US.
ISAKMP support is great in OpenBSD, and ipv6 is some of the best. They are in a transition phase with the firewall though. They removed IPFilter due to licensing reasons, and have created PacketFilter. Although I have heard great things about PF, I would not totally trust it right now. IPF on the OpenBSD 2.9 release is great (if you need some basic rules for IPF no matter the OS let me know I will hook you up with mine which are usually heavily commented 😛 ), but do not follow -current (-current source is bleeding edge/devel, -stable is just patches applied) since it has been replaced with PF.
That give you a little idea about OpenBSD? 😛
EDIT: Oh yeah, I forgot, the blowfish and daemon cop mascots for OpenBSD kick the crap out of Tux, and chuck or whatever the FreeBSD's daemon is called 😉
