As someone who is doing programming nearly 90% of my working hours, and someone who used all three major systems (OS X, Vista, Linux), I would suggest you go with a computer that is giving you highest raw metal power for the buck.

If you are really working and not playing around with desktop cpu monitor gadgets or trying out hundreds of time organizing schemes instead of doing some actual work, you will come to the conclusion that it's not that much about the editor, or the OS you are using, it's all about cpu and memory cycles you wait for while compiling, debugging, testing.

Since Macs aren't actually known for their "bang for the buck" ratio (in hardware terms), I'd go with a PC and install either Vista or Linux. Mac OS X simply doesn't add that much value to justify the price difference (plus it has its own quirks and wtfs).