Well, here's that annoying fitness answer again...
So firstly, let's look at what we are trying to achieve from doing either of these. That's calories expended. The reason we would need to bring cardio in to someones training for fat loss is to increase our energy expenditure, resulting in a calorie deficit, therefore causing weight loss.
Which does this better?
Well, which is heavier? A tonne of bricks or a tonne of feathers.
They both weigh the same.
200 calories burnt is 200 calories burnt.
There is pros and cons to both. It all comes down to the person doing it.
You have to look at which is more efficient for you.
Do you have lots of time or does it need to be done quickly? Are you struggling with energy? Do you get bored easily? Do you have fitness/performance goals alongside this one?
and most importantly...
Which do you prefer?
Because chances are, you are more likely to get it done and keep doing it if you prefer it and at the end of the day, that's far more important than then marginal difference that might be found between the two methods.
Neither are 'muscle sparing'. None of them are going to waste away your muscles. None of them are going to turn your metabolism into a furnace.
When it comes to cardio, you've got to look at what outcome you want (calories used) and how to progress it, like anything else. Start small, see what result it has and increase the amount of weekly calories you need to use, over time.
From experience, when dieting down, at a certain point the energy to get those calories burnt doing HIIT was non existent and took me forever to produce the same result. I had no power to give it a good go. At that point, I was switched to steady state by my coach. Which made perfect sense.