I have had great success with my home-built 3930k-based (Intel 6 core i7) PC with now 64GB of RAM. It was put into service as a 32GB machine in October, 2012, and has since been upgraded to 64GB and two solid state drives. DDR3 memory has come down in price considerably since I bought the 32GB upgrade last fall, and I only regret not having bought the full 64GB when RAM was even less costly in the Fall of 2012. Coupled with the RME Fireface 800, it is rock solid with Hauptwerk, and 64GB of RAM allows me to load the full PAB Gravissimo at the highest quality settings.
Since I use this machine for software other than Hauptwerk for which a PC is required, a Mac was not an option. However, I have two Macs with less memory that I use for other purposes and absolutely love them. As noted by others, they are very stable machines with far fewer issues than PCs in general.
As far as cost goes, there is no question but that by building my PC I saved thousands of dollars over a comparably-configured Mac Pro. So from a cost-benefit analysis, for me the PC was the way to go for my higher-end Hauptwerk setup, even aside from the utility of being able to use it for other purposes as well. (I realize that a Mac can be boot-camped to run Windows, but that is a concept that troubles me as it would introduce Windows issues into the Mac universe in ways that I would prefer to avoid altogether. Why ruin a perfectly good Mac with Windows?
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Now, if I decided to dedicate a Mac to Hauptwerk (and perhaps the high-end video and photo applications I use), and had $6,000 burning a hole in my pocket, then I wouldn't hesitate to buy a fully-configured 64GB Mac Pro. And because I would still use the PC and FF 800 for other purposes, I would likely budget another $2,000 for an RME UFX interface. For now, however, I'll just have to make do with the PC alternative.