There is a great disparity between buying a Mac Pro or building a new PC with Windows 8 (which Win8 is proving to be a big step forward in DAW performance) look at these figures:   
   

Windows 8 Pro System ; Intel iCore 7, 3rd Gen 3.6 Ghz Quad Core
                                 Asus Motherboard with Thunderbolt and USB 3.0 upgradeable to 64GIGS DDR 2400MHz
                                 32 GB Patriot Viper 1866 DDR 2133 MHz
                                 OS Drive for Win 8:  1  OCZ Solid state 480 GIG 540MB/s Read
                                 DAW or Video Write To Drive: 1 OCZ Solid state 480 GIG 540MB/s Read
                                 Power Supply: Corsair 1200 watt (on Mac pro:you don't know who makes it?)
                                 1rst: Data Drive Western Digital 1TB VelociRaptor 10 rpm 64MB buffer
                                 2nd: Data Drive Western Digital 1TB VelociRaptor 10 rpm 64MB buffer
                                 Asus Radeon 7970 3GB GDDR5  ( Mac Pro Video Card is truly a joke compared to this)
                                 LG BlueRay Burner and (DVD)
                                 Corsair Graphite series Computer case( Bad Ass)
                                 
                                 Total Price: $3514.88
Mac Pro Similarly equipped
(except for Apple's unknown Power Supply and whimpy Radeon ATI Radeon HD 5770 1GB Video Card

                                  Total Price:$6574.00    hmm?   Think your studio could handle an extra $3000 to spend?