This is my two tria.
BSD has often claimed the most stable networking stack on the planet, and great networking performance. So when I've looked into making my own web servers, I looked for benchmarks and found that while 5-10 years ago, BSD was better at networking than Linux, they both seem to perform as well today. BSD claims to be more secure, but security is rarely problematic on a Linux box.
If you are choosing Linux, I wouldn't choose Ubuntu for a number of reasons, primarily that they push out bleeding edge stuff without testing it. Their "stable" releases have all been very problematic at the day of release, and their testing branch included a kernel patch which permanently bricked E1000 Intel gigabit NICs. They don't have the developer staff that Red Hat and Novell have. Their packages just aren't high quality. I'd take openSUSE or CentOS to build my server on. That is a professional opinion, but it is still just my opinion.
However, if you want to look past Linux to alternatives, OpenSolaris is going to be just as secure as BSD, and probably provide even better performance. The kernel is great. IO handling is great. And ZFS is a killer file-system.
That being said, I don't expect them to switch the OS anytime soon.