I am fascinated by this claim. The last time I looked at Windows for music production was a long time ago, but back then CoreAudio beat the pants off of ASIO for real-time work. Is that not still the case?
What software do you use? Two packages that I use heavily, Logic and DP7 are mac-only.
Regarding ASIO vs. CoreAudio performance: http://www.dawbench.com/win7-v-osx-1.htm
There are a few freeware plug-ins I use that are Windows only. If I could afford to buy Altiverb 7 right now, which is OS X only, I'd probably switch to OS X (at least till Altiverb came out for Windows).
Mac is a better platform for audio but the difference is getting smaller.
It's true that they are a lot more freeware (vst) plugins on Windows. Great one like http://varietyofsound.wordpress.com for example.
The Variety Of Sound stuff is what I'm missing on OS X. I use those a lot.
Alternatively, every Mac comes with Garageband and is built from the ground up for reliable recording - if you buy a system and you hear glitching in recordings (which I've never heard of), you can take it to the Apple store and have a technician troubleshoot the problem.
I don't think most PCs face this problem (although prevalent hardware like HDMI ports is often problematic for smooth audio recording), but musicians tend to recommend Macs because the certainty that it will work out of the box has a lot of value.
Although I think this has a lot more weight when you talk exclusively about laptops, with which I indeed have had so many problems in the past that I would recommend a MacBook to any fellow musician asking me for advice, especially if he is going to go on tour with it.
The two DAWs I've assembled myself in the last 12 years were both super stable and performed really well. I don't think I would have gained anything by using a Mac.
In fact I'm on a machine that I recently built which dual boots into Windows 7 and Snow Leopard. Maybe I will benchmark Reaper in both of them and come to a surprising conclusion. If I do, I'll post it on Hacker News.