They have almost the same price on newegg and mostly have the same performance.
More aboutcore 2500k 3570k
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)
The 3570k is slightly faster (not much) but the 2500k OCs better although the 3570k @ 4.4GHz is roughly the same as the 2500k @ 4.6-4.7GHz.
Bottom line, the 3570k is the logical choice if both CPUs are the same price. Otherwise, whichever is cheaper.