OSX vs Linux
Anandtech benchmarked Apache and MySQL performance on OSX (on a dual G5), Yellowdog (on a dual G5), and SUSE (on dual Opteron and dual Xeon). Results are quite interesting. First, gcc 4 apparently is able to output much better floating point code on the G5 than older versions of gcc; if one uses gcc 4, the floating point performance of a 2.7GHz G5 is as good, and possibly better, than that of any AMD or Intel competitor. However, Opterons and Xeons still beat the G5 in integer performance. Judging by the benchmarks, Opteron is by far the best server CPU.
And second, despite the mighty G5, OSX is an awful server platform. MySQL performance collapses if more than 2 clients try to connect to the database at once. For multiple (5-50) connections, MySQL on Yellowdog Linux runs 4-5 times faster than on OSX — and this is on the same dual G5 hardware! Similar results can be seen on Apache 1 and 2 benchmarks, with the caveat that Anandtech suspects their Apache benchmark had some problems. (Of course, SUSE on a dual Opteron beats Yellowdog on a dual G5, but by a far smaller margin than between Yellowdog and OSX.)
In other words: OSX might be pretty, and it might have Unix roots, and it might have commercial applications, but it is far slower than the free competition. If you need raw performance, you need Linux.