OpenBSD has a conspicuous philosophy which involves a strong preference for C over C++, a ruthless minimalist pruning of the packages and ports tree, and an allergy to prevailing fads (see the reasoning behind OpenBSD's use of comic sans serif and CVS for examples of this).
As might be expected from a description so terse, this is a bit of a generalization. I think my preferred explanation is that FreeBSD aims to be a more usable general purpose operating system, whereas OpenBSD is more conservative, preferring to grow at its own pace and in its own directions which may not necessarily be the state of the BSD art in terms of user friendliness. A great example of this is ZFS; no free or paid OS outside of Solaris supports ZFS to the extent that FreeBSD does, but if you want to use ZFS with OpenBSD, you're outta luck - it's not even on the roadmap. Last I checked, OpenBSD had no interest in moving from GCC to Clang/LLVM either.
That being said, both OSes are free, and so is VirtualBox, so why not spin up a couple virtual boxes and give them both a try when you have the time?
In addition to this, OpenBSD develops PacketFilter, aka PF, and so OpenBSD is the place to go for the latest and greatest from that project (and many other OpenBSD projects).