Not even the smallest bit of difference in quality.
Yes the 2500K will overclock higher, but the 3570K is faster at stock speeds (5-7%).
Given the highest 24/7 overclock that I would recommend for either one (4.5-4.8 for the 2500K and 4.3-4.6 for the 3570K), they basically end up equaling out.
PCIe 3.0 is the reason to get the 3570K (and even that's not that big of a deal, currently).