The "k" at the end of the processor name means that it is fully unlocked, which means that you can overclock the CPU without any limitations, unlike the unlocked versions which only allow a maximum of what the turbo-clock allows which is usually 3.8GHz.
I classify overclocking as "above stock speed" and turbo is a stock speed. Think of turbo as a variable max speed. Raising the voltage has a greater affect on temps on IB than SB. So a small overclock is only going to be a little higher. The bigger the oc, the greater the difference in temp.
Because of the temp difference and the performance increase of IB, both cpus overclocked would be similar performance. You'd oc both until you hit high temps, for example 4.2 on a 3570k and 4.5 on a 2500k. So with the same performance (although different speeds) I'd go with IB still for lower power consumption.