Both will usually do 4.5Ghz on stock volts (1.25 - 1.35v)
Both will hit a peak of 4.9 - 5.2Ghz before hitting the 1.52v safezone (Only ever clock this high on custom water)
Both will level out at a safe overclock of 4.6 to 4.8 Ghz on 1.35v
It all depends on the batch. My 2500k is bad - it overclocks worse than most (I have to pump 1.35v to run 4.6-4.7ghz on it) - some 2600ks will perform the same, some will overclock to 4.9ghz on the same volts. Some 2500ks may hit 5Ghz on 1.4v - some 2600ks might take 1.52v to hit the same. They are virtually identical in terms of potential.
I had thought based on reading posts here that the 2600k might overclock a bit better than the 2500k. It seemed to me that I read more people with 2600k say they had 4.9GHz or so and people with 2500k saying they stopped at 4.5GHz. Of course ymmv depending on your chip, your motherboard, and how long you want to fool with the settings. The 2600k is significantly more expensive, maybe the people that spend the additional money also have a better motherboard and also spend more time working with their overclock settings.