Well, to be honest, Mac OS X Server still has quite a ways to go until it is a fully rounded operating system. Things like unaccelerated video really weigh down heavy upon the benefits of the OS.
It is, however, essentially UNIX, and so it comes very close to the performance seen with Apache on Linux.
It should also be noted that Linux support is still fairly premature for the Mac (even with recent versions of LinuxPPC) compared to the x86 architecture. Linux is just so much more robust and well-rounded in terms of support and optimizations for the x86, in my personal experience.
Because Linux is a completely open OS, it is far more scalable than Mac OS X Server. You can tone Linux down to a bare-bones, GUI-less, lean, mean, serving machine, whereas Mac OS X Server isn't *quite* as customizable.
Overall, I don't think it makes a huge difference between the two OSes. They both have their advantages and disadvantages. I would recommend trying both of them out for a while, if possible; I think that's the best way to go.