I've got used to FreeBSD now (I think :-)). Kernel reconfigs are far easier than any system of doing it that I've seen on Linux. I've managed to figure out the best ways of getting software to install, which is something that always tripped me up on previous attempts with FreeBSD/Linux.
The first company I had a 'real' (read: computer) job at used FreeBSD almost exclusively for server-end stuff (their live systems being mostly web/db serving). One of my friends works at Earthlink, where allegedly they use FreeBSD at least a hell of a lot.
At the end of the day, I think it comes down to which you're comfortable with. For me that turned out to be which I could recompile the kernel to get the hardware support I wanted, and whose package manager system I figured out first. I doubt you're going to see much performance difference, though I'm sure there'll be a few OS 'versus' benchmarks on the Intenet.
I agree with Failure about the stability argument. Every major OS available nowadays can be rock-solid stable given that decent, working hardware is available for it (well, with the exception of WinME :-)).
My advice is, try the ones you can get hold of and see. Re: Linux distributions, I would ask around which is more suited to server-side stuff.
