In Geoffrey A. Fowler’s article “How Could a Mac Pro be Slower than an iMac?” at the Wall Street Journal he explores how the Mac Pro could be slower than an iMac especially considering the
hardware configuration of the device. It basically boils down to software. If software isn’t written to take advantage of the all the cores of a Mac Pro, which it isn’t, then you will consistently end up with the results they were seeing.
Another analysis looked at cost over five years of an iMac vs a Mac Pro and discovered for the same price you could get two fully loaded iMacs which in many scenarios best the Mac Pro. In this case, the better value is of course in the two new iMacs.
However, if you are using software that takes advantage of the Mac Pro’s CPU’s and you use it a lot then the weight falls to that device.