The main difference between point-cut and flat corned beef is that point-cut beef has a lot more fat content. Flat corned beef is leaner, which means it is not as tender or as flavorful when cooked.
Point-cut beef and flat corned beef both require extended simmering in low temperatures, with point-cut beef rendering out more fat into the cooking liquid, which is what makes it tastier and more tender. The fat can also be skimmed out if the cook prefers to make a leaner dish.
Of the two, flat-cut beef is more likely to be stocked in supermarkets because most people find it more appealing.