Personally I feel hang on back heaters are lower quality and the heater is one area that you dont want to sacrafice quality. A good quality submerisble heater will last you for many years.
Salt creep is the collection of salt above the water line. Unlike FW
evaporates the salt and other trace elements in the water are left behind and they cristalize in to a solid form. As more and more of this accumulates it pushes the existing crystals up thus the 'creep' aspect of the salt. Since a hang on heater would extend above the water surface as the water level in the main tank slowly fluxuates due to evaporation (assuming you dont use a sump) there would become an accumulation of dried salt on the heater. This would need to be removed on a regular basis.