ASRock is Asus 2nd tier brand. You may run into an issue with the board and need to RMA it, but Asus is very good about shipping out replacement boards. I'd say, from my read of feedback on Newegg and other sites that youu have a 33% chance of getting a bad board your first time out and a 10% or getting a bad board after the RMA.
I have never had a Gigabyte board come to me bad but others have.
The Extreme ASRock is better for overclocking.