http://www.anandtech.com/bench/Product/203?vs=288 is a good apples to apples comparison between the i5-2500K and the Phenom II X6 1100T. The gap between the two widens with the i7-2600K. I don't know the specs they used for the test system, but Anand tries to keep it as even as possible. If you don't feel like going through all the graphs, the condensed version is the i5-2500K manages to hold on to the lead even with 50% fewer cores (more efficient computing) even in multithreaded programs, and in games it's more or less a slaughter.
As far as price/performance goes at the price range of the i5-2500K, the Intel CPU wins by far in my opinion. In the lower end, AMD is competitive, but until Bulldozer comes out there's little reason to get an AMD chip in a computer that costs more than $800. If you're going AMD for gaming, get the fastest tri- or quad-core Phenom II you can find.
The benefit of going with an AMD computer is the price, definitely. A system built around a Phenom II X6 will probably cost as much as something based around an i3-2100 from Intel, and that's a much more even comparision (with the Phenom II walking away with any heavily multithreaded benchmark.)