Do you know which revision the Gigabyte board is? Is it version 1.0 or 2.2? Typically, first revision motherboards have issues worked out in subsequent revisions. This is not to say that if you get the 1.0 revision Gigabyte board that it will have problems, but it is to say that later revisions should have all the kinks worked out and less issues. IMO, given the choice between the ASRock and 1.0 revision of the Gigabyte boad, I would go with the ASRock.
Effectively, both boards have the same specs and either one would serve its purpose; Gigabyte and ASRock are the 2nd and 3rd (respectively) mobo makers in the world. There is an equal chance of getting a bad board from either of them and both offer quality support.So, it comes down to...