I am an Asus and AsRock user and fan for quite a while. The boards are quite equal but have some differentiations. The Asus motherboard has thicker PCB and a bit more solid power design. On the other hand, the AsRock packs more features. If you are not going to mount super heavy coolers and don't want to reach the OC limit of your CPU then the AsRock is a bit better deal (although a good PSU is recommended). The Asus, however, has thicker PCB, you can mount more cooling options, can support higher overclock and can survive with a bit lower grade PSU. The downside is that it misses some of AsRock's extras. This is it in a nutshell, the choice is now yours. Good luck.
The Asus motherboard has thicker PCB and a bit more solid power design.
Actually the Asrock board has a much more solid power design than the Asus board; the Extreme4 has 8 + 4 phases vs. the 4 + 2 of the P8Z77-V LK, and bigger heatsinks on them. So the Asrock board will be a more robust overclocker.