HIIT vs. LISS
With regard to cardio in your programme, unless otherwise stated the method you choose is up to you and will depend on personal preference and/or time availability. Neither method is superior for fatloss.
HIIT burns calories in a shorter time frame, so may be appropriate if you are time-poor. If you choose to perform intervals, try not to perform them the day before a difficult leg session in order to avoid your HIIT negatively impacting your training performance. The recovery cost for HIIT is higher compared to LISS, as is the risk of injury (particularly during a diet).
LISS takes longer to burn calories, but is less taxing thus easier to recover from.