From a technical side we did a lot of research and testing in performance, security and stability. The result was that you can misconfigure every OS to be slow and instable. But based on a standard installation with common tweaks FreeBSD was more stable, secure and performant than any other Linux we tried (RedHat, CentOS and Debian).
From the financial side we found out that the ROI and TCO are a little lower than for Linux systems. FreeBSD is continuously developed and tested very well. Maintenance and upgrades are very smooth and adaptation to own requirements can be easily made thanks to the fabulous ports tree.
For me I would always choose FreeBSD as it is much more fun to work with a consistent system where you know what to do if something goes wrong. For Linux there are too many differences between each distribution which makes research a lot more difficult.
Linux is something you can use for gameservers or embedded systems or as desktop OS. FreeBSD was always meant to be a highly stable server OS and there is almost no OS which can compete.
If you don't know FreeBSD and don't know Linux you should install a common distribution (Debian, Ubuntu or CentOS) and install FreeBSD. If you fall in love with the ports tree and the consistency of FreeBSD, stick with it. If you like the precompiled packages and endless repositories, stick with Linux.