The differences between ground beef, ground chuck, ground sirloin and ground round is basically the difference in the amount of fat in the ground meat.
* Ground Beef - 73% Lean, 27% Fat
* Ground Chuck - 80% Lean, 20% Fat
* Ground Sirloin - 85% Lean, 15% Fat
* Ground Round - 90% Lean, 10 % Fat
As for as which is more tender. They are all about the same in tenderness, the only difference being in the amount of fat in each grade. The regular ground beef, with the 27% fat will probably be more juicy than ground round, but you will have more loss in weight when cooking the regular ground beef as when cooking the ground round.
Most people choose the leaner ground beefs due to diet and watching their weight. In my opinion, ground round and ground sirloin is best suited for cooking chili and spaghetti meals, while the regular ground beef and ground chuck is the best choice for cooking hamburgers.