While both LTHR and FTP are acceptable measures by which to set your training zones, there is a distinct difference between them. HR is a measure of input: the amount of physiological work your body is doing. Power is a measure of output: the amount of work your body is putting to the pedals. Measuring only input (HR) is a difficult way to determine training improvements since it is easily influenced by hydration, sleep, heat, humidity, mental state and other external (and internal) stresses. Power (output) is brutally honest in that it is a direct measure of the work you're doing (although comparing the two is a great way to note fitness gains.)