Marnie has a better AW ability and her skill is often better assuming skill levels are on par (5/5 vs 10/10, or 1v1, etc.) due to slightly more range and shorter CD (though Waltz's has longer duration, 7 seconds if both skills are maxed which may sometimes be more helpful), which is why she's considered better since they otherwise largely do the same thing. That said, if you don't have Marnie or you can CR/SU Waltz multiple times without doing the same for Marnie, you might as well use her for now, assuming you have the spare resources to level her.