A shoulder lamb roasting joint is typically a more affordable option and is known for its rich, meaty flavor. It has a good amount of fat running through the meat, which helps to keep it moist during cooking. A leg lamb roasting joint is a leaner option that has a milder flavor than the shoulder joint. A loin lamb roasting joint is the most tender and leanest of the three options, but is also typically the most expensive.
When selecting a lamb roasting joint, look for a cut that is well-trimmed with a bright red color and a thin layer of fat. It's also important to consider the cooking time and temperature for the specific cut you have chosen, as different cuts may require different cooking methods to achieve the desired tenderness and flavor.