When you're talking electric resistance heat like this, they're all equally inefficient. You're not going to see a payback period, either- 15000W of heat is 15000W regardless of how it gets from the resistance element to the room. This (and any other electric strip heater) is going to use about 2.5x as much electricity as an electric heat pump for the same heating capacity.
Immersing the heating element does reduce the risk of fire vs one with unencapsulated toaster-stype heating elements, though.
