It all depends on what you need to do with it.
If you need to use more than two displays, or install PCIe cards, or really huge amounts of memory, or plug in lots of internal drives, or use different kinds of drives, the Mac Pro has the edge.
If you need none of that, the iMac is a very nice package, and today's iMacs are compute-power competitors to some of today's Mac Pros.
If you really need none of that, a lot of folks are using MacBook Pro as a desktop Mac that almost never goes anywhere.
With Mac Pros you can add fiberoptic PCIe Cards. With iMacs you have to settle for Thunderbolt, which isn't bad, but not quite as fast. With Mac Pros you can have four built-in SATA drives.
With iMacs you are stuck with one, and based on the latest info from http://www.macsales.com/ the internal iMac hard drive is not removable! So if it dies, you are left with external drives and the built-in optical as your only choice. With Mac Pros, you can expand your RAM more.
With Mac Pros the processor is frequently more powerful if your software desires it, and your graphics processor can be too if you desire it. If you don't think you'll need it, after examing the machines at the Apple Store, the iMac is a great machine.
You are more likely to outgrow an iMac and it is harder to build one and keep it cool in confined space than with the parts and design of workstation.
I always feel if you have to ask, you don't need a workstation. But if you want 24-48GB RAM and 6-cores you do. And having 5 drive bays of course.
If you plan to work with CS5.5 or 6 with video and graphics editing but even FCP-X can be done on any Mac.
You can't just ask wtihout some parameters - for all we know you want to use Facebook and such and would do well with an iPad