Diana is simply much better. I mean, she's been trained by the best fighters on Earth since childhood. She learned how to break a man's arm when she was still a toddler. She trained every damn day until she was twenty-two and then she became Wonder Woman. Then she trained with Batman, Shiva, Black Canary, Starfire, Richard Dragon, and dozens of other martial artists to increase her repoitoire.
And unfortunately for Robin, Wonder Woman's I Ching saga is now back in continuity. That means all the Pre-Crisis tales of WW being powerless and blowing through entire armies using her training is viable proof of her skill.
Robin isn't anywhere near her skill level....
He's only had about a half decade of training (he became Robin III at age 13. He's stated to be about 17 to 18 right now). Every single one of his teachers have already taught Diana to a much longer length.
I mean, really, this is a no-brainer.
And Black Canary is a top-tier fighter now. She's arguably better than Nightwing due to her new training as she followed the path of Shiva. Robin doesn't hold a candle to her.
I would say that Robin is a better fighter than Wonder-Girl though...