Which exactly is more powerful? The 570M has significantly more cores but a lower clock frequency. I know that a raw comparison of these two generally isn't enough to determine power, but this is within the same generation. I can't find any benchmarks anywhere comparing the two. Would the 570M have higher overclocking headroom?
The 570 is more powerful because it has more cores working on the same process than the 560. Even with a lower clock frequency the additional cores make shorter work of a graphics task than a higher clocked 560. The OC'ing headroom on a given GPU is individually determined by the processor. Some 560s OC better than other "identical" GPUs. The 570 will probably have better potential to OC, but it will depend on the GPU in your machine.