Linux and FreeBSD are two popular open source operating systems. Many Linux users want to know more about FreeBSD. Linux is much popular compared to FreeBSD. You will find FreeBSD users saying FreeBSD is more secure, stable than Linux.
If you do a Google search you will end up with lot of discussion over this. But most of this discussions are not useful. Most are just arguments with no solid proof. Those who use/love FreeBSD says its superior to Linux.
Linux and FreeBSD use almost similar commands. For example ls, ps, top, du, df, etc... all works both in Linux and FreeBSD.
FreeBSD have port system. This is easy to upgrade all installed software.
Linux have yum, up2date, apt-get depending on which distribution you use.
FreeBSD is told to be more secure than linux out of the box.
Linux, you can secure it if you know what you are doing.
I have both Linux and FreeBSD servers. I never found any security issue with Linux servers. You should keep the software uptodate. It may be insecure to run Red Hat 7 or 9 as its outdated software. In my opinion, if you keep the software uptodate, you are secure.
Some usefull links
Benchmarking BSD and
Revisiting FreeBSD vs. Linux for MySQL
MySQL Work better on Linux. Benchmark - FreeBSD 4,6 and Linux
Linux: vmsplice() versus COW
Linux: OS of choice for 78% of the world's fastest supercomputers.