With HIIT cardio most of the calories being burned come from muscle glycogen, HIIT will speed up the metabolic rate, & u produce more GH when u do HIIT, but HIIT can be counterproductive if u over do it, that's why is best to keep it to 2/3 sessions a week.
The lower the intensity, the greater the percentage of energy derived from fat....the problem with low intensity (long cardio sessions) is that it will get ur body into a catabolic state and it will start to break down muscle tissue for energy, so it's better to do a higher "shorter" session.
U can do HIIT twice a week & add some lower intensity sessions as well throughout the week.