Based on what? The only people who supposedly know what happened are the ones allegedly in that room and the only evidence is their testimony. Judge gave a written statement and he is not going to talk to investigators. That leaves Ford and Kavanaugh who will say essentially the same thing to the committee as they say to investigators. An investigator might be able to tear down one or the other's story more easily than the hearing would, but we are never going to know definitively what happened. They are evaluating two stories and the outcome is deciding which on they believe more. An investigation is more likely to be bad for Ford because the FBI can ask her hard questions and identify inconsistencies in what she is saying that a senator won't be able to do for fear of looking like an ass on TV. But if there aren't any inconsistencies, then the same thing comes out in the hearing.