Based on my experience, I've found that running Digital Audio Workstation (DAW) software under OS X makes my life much, much easier _if_ I have access to audio and MIDI hardware that have a proven track record under OS X. And by "easier" I mean much less hassles with drivers, latency, and other potential problem areas, along with having a user-interface (UI) that's cleaner in appearance and easier to work with. Typically, I can get more done, in less time, with less hassles, using OS X vs. Windows DAW software.
However, Windows DAW software and Windows itself has improved in recent years, and most all commercial DAW software is available cross-platform these days (although Apple's Logic is a notable exception). And, at this present moment, you can buy PC (quad-core) laptops that are more powerful than Apple's current (C2D) offerings. If you are planning on using lots of effects, wanting to work with a great number of tracks, etc. you will usually find yourself wanting/needing all the CPU that offers a good amount raw processing power you can get your hands on.
I'm considering purchase of a new laptop to replace a 2.75 year old MacBook sometime within the next year or so. Personally, I'm willing to wait and review the specs of Apple's new 2010 MB and MBP releases, before I even consider buying anything. But then I'm in no great rush, as I don't need something right away. What I have still works fine for my needs, but then I only use the MacBook for mobile recording, typically of four tracks of less, and not for final production -- the heavy lifting is done on desktops.