i5-2500K and the i5-2550K are the same except that
1. the Core i5-2550K does not come with integrated graphics. Disabling the integrated graphics may allow it to run cooler and possibly will reach higher overclocked clock frequency.
2. The base and the turbo clock of the i5-2550K is 100 MHz higher than the i5-2550K. The core i5-2500K has a base clock of 3.3 GHz while the Core i5-2550K has a base clock of 3.40 GHz.
The base clocks does not matter anyway, if you are overclocking it. If you are not overclocking, you may better buy the core i5-2500 or i5-2400 and save some money.
The basic thing is that the core i5-2550K strips of the integrated graphics, so you have some more room to make overcloking faster.