Well, just because I like something more, doesn't mean it is better. Sort of like a car, I may like a '64 Impala the most, but doesn't mean it is better than a Lamborghini. But as for Visas winning, she had for more experience in dueling one-on-one saber dueling than Qui-Gon, he lost his first real one-on-one.
I don't know..
From all we know she means nothing. Nihilus took her for her special power to see people and no other reason. She probably had no impact on Nihilus at all, and he didn't kill her the second for the same reason he didn't kill her the first time.
However Kreia does speak about Nihilus Sion and Visas as dangerous opponents once. Maybe Visas is more powerful then I think.. QGJ isn't that good, so she has a chance, although i'm not sure if she wins or not.