I think it depends on the persons.. There are Hetero parents who suck ass, single parents who fail and i would bet that there are gay parents who will fail big time too.. Parenting skills have nothing to do with the sexual orientation or the structure of the family...
If the gay parents are child rapists and the hetero parents are not then clearly the hetero parents are better. If the gay parents are not child rapists and the hetero parents are then the gay parents are better.
Otherwise I don't really see the difference. People make a big deal of a child needing both a mother and a father, but I think a child can be equally well adjusted having two mothers or two fathers, provided, for example, a son has a male role model somewhere in his life, if not in his parents then through an uncle or through a family friend, and vice versa for a daughter. I think the key issue is that children no longer need their parents to be a stereotypical couple, families are falling apart more often than ever before so a child won't neccessarily have that father or mother-figure anyway.