As I understand it, cattle is graded ONLY on the ribeye.
So, if there's enough marbling in the ribeye (rib roast) to be graded prime, then the entire animal is graded prime. If the ribeye is graded select, then the whole animal is graded select.
How this matters is that there is a definite possibility that the brisket on a certain animal that's graded select or choice COULD be the same or better brisket than the brisket on another animal that's graded prime, since they don't consider the brisket when grading beef.
I'm not sure why a brisket would have a lot more marbled fat than the ribeye on the same animal, but I do know that just because a brisket is graded as "choice" doesn't mean it won't have "prime-like" brisket fat marbling.
But....GENERALLY.....a prime brisket will have better marbling than a choice or select one and that means better flavor and moisture.
Big JT's Smokin' BBQ Competition Team
"There is a freedom that cannot be taken from you. There is an inner freedom that nobody can ever deny you of, even if they lock you in a cell and block you off from the world. Seek that freedom."