They are not better - both are needed for maximal odds of a good outcome for the kid.
Children raised by a single mother are 2-3 times more likely to: be held back a grade, drop out of school, become mentally ill, become an addict, join a gang, commit suicide, commit murder, go on welfare, have an illegitimate child, be drunk.
For two parent married mother father families, the odds of any of these for their kids is about 10%. For a single mother's children, it is 25-30%.
And most telling, per the Regernus study, two "married" homosexuals raising opposite sex children have kids who have WORSE outcomes on nearly all of these metrics. So a boy raised by two lesbians does worse than raised by a single mother, as is a girl raised by two homosexual men. The life outcomes are almost as bad as a single mother for girls raised by lesbians and boys raised by homosexual men.
This means it isn't one parent or the other that is better - it is the balanced, stable example of one of each that maximizes their odds of success.