I would recommend the 3570k. The 3570k will run slightly faster at the same clock rate and it is more power efficient. The drawback is that is overheats faster, causing it to have a lower max overclock. You can overcome this drawback by getting a better thermal paste. I have not seen a lot of reports on how well 3570k overclocking is with better thermal paste, so I don't know if you can reach the same frequencies as the 2500k. Even without adding new thermal paste, either processors can overclock over 4.0 GHz.