Generally speaking Gigabyte is a more solid brand. But over the past couple years ASRock as come a long way and is now a viable option.
Both boards are very similar, and are well built. Either way you go will be a solid choice. I personally would probably go with the ASRock between those 2 because of the SATAIII ports, the price, and honestly (because I have a windowed case) the physical look of the board. I don't like the light blue that Gigabyte used on their board.