I was the owner of a 2009 27" iMac and just recently sold it to buy a Mac Pro. I'd suggest you to get one because I couldn't be happier with the switch but it depends on what's most important to you.
Personally the HDD in my iMac was starting to make noises and it annoyed me to no end to have to bring the whole computer for repair if anything was wrong with it. With a Mac Pro you can easily change parts if anything is wrong or that you want to upgrade. Even the CPU is a reasonably easy upgrade in the 2010 Mac Pro; see the threads about people upgrading their base quad core to a powerful 6-core for cheap in the forums. If you plan to game you also get a better graphics card (with the 5870) that is also upgradable.
The biggest problem with the Mac Pro though is its price. With an iMac you'd have two 27" screens while for about the same money with the Mac Pro you'd only have your ACD unless you want to buy another one. You'd also get, if you stick with the base model, a slower computer, although they should be pretty close. But contrary to the iMac you can easily upgrade it later and make it faster with a CPU swap, internal HHDs and SSDs, more RAM (you can get 32GB for a reasonable price on the Pro, 16GB only for a reasonable price on the iMac).
So there you go!