80% lean ground beef contains more fat and calories than 90% lean ground beef, but it can also be more flavorful and juicy. It's a good choice if you're cooking burgers or meatloaf, where the fat content can help keep the meat moist. On the other hand, 90% lean ground beef contains less fat and fewer calories, making it a healthier choice. It's a good option if you're looking to reduce your overall calorie and fat intake, or if you're using the ground beef in recipes where the added fat isn't necessary.