I just was shopping for AM3+ boards. I looked at both of those and thought they were excellent. Then I still went with the Asus M5a99FX Pro R2.0 and couldn't be happier with it. The power phasing may not be the 12+2 of the ASRock or the 8+2 of the Gigabyte, but the digital regulation on the Asus board really does make a giant difference. I have a M5A88-M right now and the default settings with just the multi increased yielded a better stable OC than I could acheive with the same chip and careful tuning on the '88 mobo.
Otherwise, I would go with the ASRock. 12+2 power should yield better OC potential than the Gigabyte, and I have generally seen the ASRock used in more extreme benchmark scenarios than any Gigabyte mobo.