I really think Momule hit it on the head pretty good. I havce built several DAW's for friends and all were PC based. I like the Mac's but you your options for upgrade are somewhat limited by being an Apple product when compared to a Wintel.
First, I will say that I use to work at Apple (10 years total), so my opinions are not without bias. That said, I would argue that the upgrade path is limited. Sure you can't toss a new processor in, but guess what, if you're not a gearhead and willing to putz around with the OS and BIOS, you may never get an upgraded processor working either. That aside, very little requires an upgrade. Price point wise, you're pretty close and OS wise, the difference is night and day (I'll take Mac OS X over XP any day of the week ... I use both daily and Sun OS quite a bit and for those who use multiple platforms will probably choose Mac OS X).
Software-wise, there aren't any limitations, although you might have to use a product you haven't heard of before. On the DAW front there are more options since Mark of the Unicorn's Digital Performer and Apple's Logic (formerly Emagic) only work on the Mac.
I'll leave you with one small tidbit that was told to me while I worked at Apple. When Emagic was purchased by Apple and they chose to discontinue the Windows version a lot of people got pissed at Apple. However, in talking with a former Emagic employee who came to Apple he mentioned that in their support costs, 80% of the issues were generated by Windows owners and 20% were Mac owners. That sound fair, when you consider the number of PCs versus Macs. However, then he told me that 75% of their user base were Mac owners. So, 25% of the users were costing 80% of the support costs. Now how accurate these numbers are, are anybody's guess. But this goes along with what I've heard at some other companies that have cross platform products (that is, support tends to take longer and be more esoteric on Windows).
So, I'll leave the Windows vs. Mac decision to you, but I honestly think that if you use Mac OS X for any time at all, compare costs (without building one yourself) putting in all the same features on both you might be surprised at how closely the costs really are...
Wayne