I can say from experience that Asus motherboards are very overclocking friendly and they have a lot of options including preset overclock settings that make for a very easy medium overclock.
I would say that Asrock would be similar in overclocking ability but may not have as many options.
Asus is Cadillac and Asrock is Chevy -- same company makes them but asrock is supposed to be the cheaper brand (though many benchmarks I've seen it performs the same or even a bit better)
Would make sense though that the higher end "brand" has more bells and whistles
Asus is just a great company to buy from, my led display is them, my vid card is them, had a couple mobo's and no problems with any of it.