which one of these has the better chance of being accepted based on that fact?
Hypothetical question? Neither, it depends on the application and the applicant pool.
I think legacy helps only on the margin, and only for certain schools with ED applications. I was told by the alumni office that Penn likes legacy candidates who apply ED. That said, I know plenty of Penn grads whose kids did not get in ED. First generation probably a bit more of a boost but again, depends on the school, the pool of applicants, and the strength of the application.