i have to say go with the asrock. when i did my research for which board to get i found that the asrock boards were on average achieving much higher overclocks than any other brand.
so i bought a z68 asrock extreme 4 and can overclock my sandybridge to 5.1ghz stable (albeit too hot with my air cooling.)
edit: and this may not be a selling point for you, but it was for me: the asrock boards can mount heatsinks from other chipsets without needing a different bracket. i reused the heatsink off my socket 775 setup. saved me 30 bucks.