I would go Gigabyte over ASRock. MSI is coming out with a motherboard that has a new chip in the end of October. This chip will allow many different multiple GPU options. The board is called the Big Bang, unfortunately I doubt it will come close to this price range.
Gigabyte has certainly earned a recent reputation for producing quality motherboards. However, ASRock has stepped up their game in the past year providing top notch mobos for both the Skt775 and Skt1366 platforms. I am currently using 3 ASRock mobos (HTPC, NAS, and mykids computer) without any issues or problems and can easily recommend ASRock as a stable and value added foundation for any machine.
Given that the Gigabyte and ASRock you linked are for the same price, I would suggest making your purchasing decision based on features and independent website reviews. Not to discount the user reviews on Newegg, but they can sometimes be suspect. Personally I prefer an ATX sized mobo over a micro-ATX unless I have specific reason to use a micro-ATX.
Regardless, I believe that either mobo will perform to your expectations.
But if it comes down to two comparable motherboards at a similar price, the next discriminator should be board layout. Will a full size GPU cover the SATA ports? Will an installed PCI card block the air going to the GPU? Will any motherboard components around the CPU limit heatsink choices?