A PC is generally faster than a workstation, so expect to see similar performance when considering the specs involved. Workstations have a major advantage however when it comes to redundancy and error correction. If you are doing multiple thousand dollar contracts you dont want to have the computer spewing out jibberish or crashing instead of outputting a beautiful final product, and that is why you purchase workstation parts.
The other reason for a workstation is that (assuming you are made of gold) you can get much higher performing parts than a desktop on the upper end of the spectrum. For example, a desktop is limited to a single CPU, but most workstation boards can do 2-4 CPUs, and 2-4 old CPUs will still kick the butt of a single modern CPU when it comes to raw number crunching. Workstations can also do much more Ram than a desktop (though the LGA 2011 has plenty of space for most applications today), and tend to have more/better built in RAID controllers which cost a pretty penny if you have to buy one separate.
Workstation GPUs have specific hardware to accelerate 3D content creation work, but when it comes to raw cost vs performance a standard GPU will do just as good as a similarly priced workstation GPU. However, again, a $700 workstation card is going to make any 590 owner cry when it comes to exporting things that take advantage of the workstation-specific hardware. Cost per cost the 'civilian' equipment is faster, but the workstation stuff begins where the high end gaming stuff ends, and only goes up from there.
All that being said, for most people a production oriented desktop can provide more than enough power for their needs, and workstations are mostly for people who are doing 24/7 rendering, instead of working on a project that takes a mere 6-8 hours to export.