After overclocking there is a huge difference between the CPUs.
i5-2400 3.1ghz / i5-2500k OC 4.6ghz = 48% clock increase.
For overclocking you'll want some aftermarket cooling and a P67(or)Z68 board.
What are your uses? i5-2500k is good for anything CPU intensive. i5-2400 will be better for a machine you want limited power usage/heat in but don't need super CPU clocks.