You're comparing different price brackets. In the ASRock Z77 Extreme6 price range, other contenders are that I'd recommend are the Gigabyte GA-Z77X-UD3H or the MSI Z77A-GD65. They will all suit you well. The Gigabyte one would be the one I'd pick just for the better build quality. But if you like the Extreme6, it will not disappoint you. I have it and I'm definitely very satisfied with the Extreme6.
I already have a MSI-z77-G43, and it does everything it's suppose to. I'm just really unsatisfied with it when it comes to overclocking so I was looking at these two boards. I've heard alot of good things about gigabyte too maybe I'll look into some of their boards.
So the only thing thats differant is the price? I looked at all the specs, their all nearly identical. I know Asus's are pretty top-notch when it comes to overclocking and just in general, but I've heard mixed reviews about asrock.