Unlike the i5 2500K/3570K, the i7 3930K/3960X are both hexacore (6 core) processors with hyper threading whereas the i5's are quad core. Additionally, moving over to the LGA 2011 platform which is what the 3930K and the 3960X are on, you also get the benefit of having a lot more memory DIMMS as most of these LGA 2011 boards support 8 DIMM slots. You also get more PCIe lanes as well.
In terms of pure overclocking, go with the i5 2500K. AFAIK, the easiest processor to overclock is the i5 2500K.