HIIT vs LISS
First lets explain the two. HIIT - high intensity interval training, is alternate bouts of high effort work (near maximal effort) followed by longer periods of low effort work or even rest. LISS on the other hand is exactly what is says, low intensity steady state cardio (eg walking) this is probably the one you see plenty of the old school bodybuilder doing in the gym on a treadmill, sometimes inclined.
Now the question which lots seem to ask is which is better for fat loss. As always the answer is it depends. If you're pressed for time and prefer the jacked feeling, plus your already in decent cardiovascular shape, don't have a high injury risk then HIIT might be the one for you. The downside to HIIT is that if done properly it's incredibly taxing on the body, and so if your in a heavy schedule, training, work, family etc then this added on top will only compound stress and will certainly have a negative impact on your recovery process, thus affecting your lifting and also your daily activity patterns. This could even reduce your overall calorie output in the end. Plus its not for everyone
The up side to it is that per unit of time compared to LISS is does burn more fat. Meaning if you spend the same time doing both HIIT is more efficient. So if you're pushed for time then it's definitely better.
I used to be very against LISS. Probably for personal reasons, i'll admit now im wrong in thinking such a way. It isn't as detrimental to muscle growth as first thought. In fact i use it alot with our combat athletes. Its an excellent form of recovery, circulation and nutrient delivery on lower intensity days or in the evening after a tough days training. It also is very useful in burning calories without affecting a training programme. However it can be time consuming. We like to recommend low impact forms like a cross trainer, bike, rower or incline walk for example. These reduce the impact on joints and possible stress.
They will both burn calories. They both have their place in a training programme (when needed). It ultimately comes down to choosing one that wont ruin the rest of your plan, and the one you have the most time for.