My local Cosco carries both prime and choice. The full packers are usually prime and actually cost less per pound than the flats, which are typically choice.
I am still working on trying to cook a full packer where both the point and the flat are juicy. Usually, I get the point right but the flat ends up a bit dry.
The best brisket I ever cooked was the first, which was a flat cooked with charcoal on my Weber kettle. I am not sure what I am doing wrong.