If you want a one word reply it must be: Linux
FreeBSD is a very nice OS but in my humble opinion it is not production ready unless you really feel like spending lots of time tinkering.
FreeBSD is suffering from a huge lack of "eyeballs" compared to Linux. Over the past years FreeBSD have missed many release schedules and you just get the impression that there are not enough FreeBSD developers out there to deliver the functionality and stability which we see in Linux distributions such as Redhat.
An important production issue is lack of official hardware support. Most vendors of servers, disk controllers, etc, will officially support Linux but not FreeBSD. The same goes for software. It is sometimes diffult to find FreeBSD support for your favourite configuration management tool, virtual machine hypervisor, etc. etc.
Many databases, such as MySQL, work better on Linux because the majority of the developers, consultants, etc. work on Linux. So the Linux platform gets tested, documented and optimised much better.
Package management on FreeBSD is less mature than Linux and due to lack of official FreeBSD support you often end up compiling your own packages (and kernel if you have a bad day) and maintaining your own package repositories. This really slows down patching/vulnerability management.
So in many FreeBSD production environments you may see low productivity and delayed projects because your staff will be spending time on debugging and fixing some low level critical issue that just works out of the box on Linux. Well, that is my experience. Your mileage may wary.
May your OS be with you!