At stock speed the Intel® Core™ i5-3570K will outperform the Intel Core i5-2500K. In the terms of highest clock speed you can reach higher clock speeds with the Intel Core i5-2500K. But due the IPC improvements on the Intel Core i5-3570K will perform at about the same level as the Intel Core i5-2500K. In the end when overclocked you will get just about the same performance from both of them. In that environment you will also pick up PCI-E 3.0 and native USB 3.0 support from the Intel Core i5-3570K.
In the end you can overclock the Intel Core i5-3570K up to around 4.2GHz or 4.3GHz and get the best of both worlds.