Well, I'm uising both a mac and pc at work. IMHO, windoze has many ram intensive system overheads, coupled with the fact it cannot address more than 4gig of ram on a non 64bit system. the macpro desktop on the other hand has little overhead and can address up to 16gigs of RAM.
On the usability front, a laptop (either a mac or windoze) ain't gonna differ by much since rams and screen estate should be the main criteria of consideration, followed by rendering/processing power (which are on par and relative to processing mhz than platfrom)
the only issue is response time. On a mac, it is definately more responsive as windows has a bad habit of trying to pre-fetch or pre-read files before you execute it. Hence opening hugh jpegs/psd and vectors can be very laggy versus a lightning fast response on an OSX. Pass that phase, applying filters and blurs has almost negligable difference for both mac and pc.
On the cpu overheads issue, windows has more bloatware (services and unnecessary "visual style" eyecandy theme) which requires user tweaking before it can be optimise for intensive usage. osx off the shelf doesn't require much user intervention.
Now if you ask me whether which platform I would use, I can only say it depends on the usage. Video editing, mac gets my vote for it software and codec plus response time, for 3D work PC gets my vote for its optimise and easily modifyable graphics drivers and plugins/speed/3 button mouse and shortcut keys (as I use maya/3dsmax). Editing pictures, mac gets my vote for its response time when i need to sift through thousands of images. I don't play games so I cannot comment on that.
Personally, I would only get a Macbook for it's price to reponse time for CASUAL work, Windows laptop for it performance to price ratio and SERIOUS work. And macpro desktop for it's value (dual 64bit dual core xeon at 4k is hard to beat) which with the upcoming OSX leopard, should be great for running XP on the system.
I say this because a 13' macbook screen is too small for any real work seriously and for $3k i can aquire a performance laptop with 17" screen estate at 1680x1050 resolution. where I would have only gotten an entry level MBP.
Ultimately, it's about the USAGE and YOUR BUDGET.
