I give Prometheus the slightest edge for having Oscar-winner Theron in it. That's just the one thing Covenant needed. A Theron, a Crowe or a Blanchett. Just one.
Fassbender and Rapace are in both, so it's pretty much equal there. Even though Rapace is probably in a smaller part in Covenant and Waterston is taking her place as the female lead. Waterston should be catapulted in popularity after FANTASTIC BEASTS.
Now let's take the other groups of four: Theron, Marshall-Green, Elba, Pearce. Theron is A-list. Marshall-Green was and still is unknown. Elba at the time was just taking off in the big blockbuster world, with Thor preceding this movie. Pearce is a great, semi-known character actor who led MEMENTO.
Now let's take the remaining three from A:C: Crudup is probably on the same level as Guy Pearce. They're pretty much at a tie. He led ALMOST FAMOUS, which like Memento, was a great film from long ago from a prolific filmmaker. McBride is inspired, as is Bichir. Bichir is an Oscar-nominee but is in a lot of lesser known films still, but still has pedigree from HATEFUL EIGHT. Overall those two are more known than someone like Marshall-Green, but nobody in this cast tops Theron except perhaps Fassbender, but he's a holdover from the previous entry anyway.
I vote Prometheus, marginally.