We've been trained to think that USDA Prime beef is better. So a Prime Chopped Steak or Prime Sirloin Burger - from Morton's no less - sounds like it would be better, too, no?
I am not so sure and here's why.
USDA Grade measures the % fat in a single muscle and that muscle is not the one making up your burger.
USDA Grade is not the only indicator of flavor or texture. Breed, diet, growing region, husbandry standards, and whether and how a butcher has aged the beef will all make a huge difference, too.
Let me explain a bit more, noting there are further details on the USDA site and in my head.
Point 1. USDA Grade is primarily determined by just two factors.
The % of marbling. Marbling refers to the white speckles of fat inside the muscle (as opposed to the fat trim on the edges). To determine the % marbling, the USDA inspects the muscle between the 12th and 13th rib. (This section is usually trimmed into a Rib-Eye steak.)
The age of the cattle. Younger cattle (<30 mo.) will generally produce more tender meat. The inspector is trained to review certain skeletal features and the color of the meat to determine the approximate age of the cattle.
Once these criteria are accessed, the inspector uses a roller to mark the entire cattle as Select, Choice, Prime (or some other classification). As long the cattle is estimated to young enough, the higher the marbling score, the higher the USDA grade with Prime being the highest. [For all you wonks, you can find the USDA calculation here.]
There are 11 sub-classifications between Standard and Prime grades including 3 in the Prime grade alone (Abundant, Moderately Abundant, Slightly Abundant - here are some images).
Further, the % marbling in the rib-eye does not tell you the % marbling in the other sections that are typically used to make ground beef.
Point 2. The amount of marbling and age of cattle are not the only keys to flavor and texture, anyways.
The USDA grading system does not take into account the breed of the cattle, where it was raised, the diet, or whether and how the beef is aged post slaughter. All of these impact flavor and texture.
Indeed, quoting Harold McGee from his book "On Food and Cooking," “...the current consensus among meat scientists is that fat marbling accounts for no more than a third of the variation in overall tenderness, juiciness, and flavor of cooked beef.” [Italics are mine.]
I have personally tasted burgers (and steaks) made from cattle that acheived only a Select or Standard grade - meaning they had little to no marbling - that were absolutely full of flavor and delicious.
The bottom line: The label “Prime” Burger does not tell you how much fat is in your burger nor does it tell you how that burger is going to taste.
This isn’t to say that such burgers might be delicious – they may very well be. It’s just that I wouldn’t pay more for one labeled as simply as “Prime.”