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.
every brand has their own issues. my gripe with gigabyte is low quality components, no vram heat sinks, bad usb controllers, sub-par audio, etc. they make good higher end boards though. asrock boards are really well balanced for the price and are more competitive for lower priced boards