Leaving that minor detail aside, I (unsurprisingly) think FreeBSD is much better than any variety of linux for this purpose. The first rule of running servers is "apply security patches", while the second rule is "don't break things" (some people reverse these two...). FreeBSD works with a concept of "stable branches", and as a general rule, FreeBSD does not break APIs or ABIs except across major version upgrades. In contrast, security patches to linux have been known to change kernel/userland interfaces, as well as kernel APIs -- either of which can result in system administrators needing to run around fixing things after applying a security update.
This is even more relevant in the context of cloud computing, where the patching (or not patching) of the operating system is performed by someone (e.g., Amazon) other than the people actually using the virtual machine instances. I don't think Amazon has really figured out what to do with linux here; but for FreeBSD the right answer is simple: Update images as soon as security updates are released, secure in the knowledge that doing so won't break anything.