After mastery of the Bakusai Tenketsu, the normal final development of that family of construction based martial arts is the Seirensho Tenketsu. Rather than causing an explosion of a target, it vibrates it to the point of melting, then separates and rearranges the atoms into desired metals. This requires a very high degree of tolerance to heat, as well as a very fine control of the affected elements themselves through infusions of ki. As part of the training, one must be able to use the breaking point on liquids and to be able to separate heavy elements from light ones in a solution. It has little direct combat application, but Ryouga had hoped to be able to use it to heat water headed towards him, thus not triggering his curse (after being cured, that became redundant).
The difference between the two lies in the very nature of their development. The Smelting Point was developed for two things, to enhance user’s options in close combat and to manipulate the battlefield. While the Metal Fatigue Fist was develop to defeat difficult defenses and to provide openings, rather than directly cause damage. In a broad sense the Metal Fatigue Fist is used to 'support' while the Smelting Point is used to 'maneuver.'