not really a proper comparison.
corvette vs porsche may be a better one.
I'm seeing on forums that the prosumer HP gets within about the same price as a MacPro only cannot match the MacPro's proprietary configuration at any price right now with PC hardware. The difference in price is small even if the HP offers greater 'right now' expandability (that you'll never take advantage of, like supporting 512Gb of 1866mhz DDR3 ECC RAM because it costs $120k) and won't be bested until more external Thunderbolt devices become available.
The bitchin' Camaro builders can build a powerful rig that may come close to a MacPro in power and save $600, but when all is said and done they're going to be Windows users driving a homebuilt PC of questionable reliability and no warranty.
Dollar for dollar, nothing beats the MacPro right now as a prosumer rig that mortals can afford. Best part about it is that it runs OSX Mavericks and can also run Windows in VM about as fast as the comparative PC can run it natively if you simply must have Windows around, like I do.