HIIT vs steady-state cardio: which is better?

The honest answer is that they do different jobs, and the better one is the one you will keep doing.

HIIT is time-efficient, pushes your top-end conditioning hard, and is genuinely uncomfortable. Two or three sessions a week is plenty — it is a large recovery cost. Steady state builds the aerobic base everything else sits on, is easy to recover from, and can be done most days. It is also far less likely to interfere with your strength training. If you are short on time, HIIT. If you are building a base or already lifting heavy several times a week, steady state. Most people do best with a mix: one or two hard sessions and a couple of easy ones.