I don't think Peter could have bested Sirius in a duel. And I don't think Peter believed he could either - which is why he chose to do what he did. He had his wand behind his back and either Sirius didn't realize that or didn't think Peter's wand behind his back was a threat to him. After all, Sirius, in his unfortunate arrogance, believed that he was a much stronger wizard than Peter, and wouldn't have thought that Peter could best him either. That was why, when he was taken by the Aurors, he was laughing so maniacally.