If you spend any time on free software websites or forums, you have undoubtedly heard both sides of the argument of Linux vs. BSD. Each side claims to have advantages in particular areas, so it may be difficult for you to easily tell which one is actually better. Before you even begin to decide which one you want for your dedicated server, you should know exactly what they are and how they work.
The terms Linux and BSD are actually quite generic. It would be equivalent to saying Honda vs. Hyundai. Both are very similar, and unless you compare specific car models, you will probably not be able to find a significant distinction.
The term Linux does not refer to an operating system but rather the kernel that powers several operating systems called Linux distributions. BSD was once an operating system, but only its numerous descendants survive, each with their own variations.
Because there are so many variations of both Linux and BSD, the only way to effectively evaluate them is to pit one variation against another. In other words, it may be equally valid to compare Red Hat Enterprise Linux to Mandriva, as it is to compare Ubuntu Linux to FreeBSD. While the kernels may have significant differences, both Linux and FreeBSD use much of the same free software to control their various functionality.
If you were looking for a simple answer to tell you which is better, I apologize for not being able to honestly provide it. While I am biased toward Linux as a personal preference, I would not be able to tell which website is powered by a Linux server over a BSD server unless the owners made the information public. Certain versions may suit your needs more than others, so plan your usage accordingly.