Depends what your goal is. If you really want/need hyperthreading then the i5 2500K is already out of the question. If this is simply a gaming machine though, the 2500K should be sufficient.
On amazon the 3770K is priced slightly higher then the 2600K, performance wise they're similar but the 3770K has lower heat output and power consumption due to it's 22nm design. It also have a better onboard GPU. If these are not a factor for you then go with the cheaper 2600K.