get the 2550k if you can, 2xxxk series will overclock higher than the 3xxxk series. that is because intel for some reason used cheapter metal conductor on the cpu where the heatsink touches so they run hotter (bad for overclocking)
3570k is more efficient and runs cooler. Although, as said before, it will not clock as high, but it will still OC quite well. I would go with the newest chip.
Also, if you have a motherboard that supports it you will only get USB3 and PCI-E3 support with the 3570k. So unless you are going with extreme overclocking ability it's probably the better choice.