HIIT, definately.
I used to do cardio 6x per week, for 45-60 min. Gains came slow - both fat loss and muscle gain, even when on gear. Now I do it 3x week, 30 min HIIT on treadmill or StepMill, and I can maintain my BF, gain muscle (Test Prop helps with that though....), AND my diet isn't squeaky clean.
After the New Year my diet will go back to squeaky clean and with HIIT I assume I'll lose BF.