I have used both Mac and PC machines over the last 23 years as a professional designer and illustrator. I can honestly say that I have found no real difference in reliability between them. I have always commissioned bespoke PC builds rather than buying off the shelf, that way you get what you want and can buy the best components available at the time. A bespoke PC will still be cheaper than an equivalent Mac. I currently use a top end imac running Parallels and a Bespoke PC for some jobs. Both work well, are fast and stable.
Historically, the only problems I have had have been hard drive failures, on a Mac G4, a PC and a recent warranty replacement on the hard drive on an iMac. My business partner has had hard drive, screen and disk drive failures on the same spec imac bought at the same time, from the same supplier, so I think reliability is largely down to luck! Macs look great but you pay for it.