As a personal trainer I get a lot of the same questions a lot, so I figured a bunch of you out there might have some of the same questions too! One of the ones I get the most is "What's better? Long steady state cardio or high intensity intervals?" 
The answer is a bit more complicated, because it depends on your goals, and what's going on with your body and lifestyle. 

Long Steady State Cardio: 

About 65% of your HRmax (220-age) so for me 220-29 = 191 for my max and 65% would be 125 beats per minute. Performing at this range for more than 20 minutes

LSS has a great benefit for your aerobic capacity, it's also really great for your cardiovascular system. By performing a this rate, your left ventricle fills completely (this doesn't happen during HIIT) and this allows this ventricle to stretch, becoming larger, with a larger capacity to hold more blood, meaning your resting heart rate will drop. This means overall your heart is more efficient and doesn’t have to work as hard, yay!
LSS tends to take more time, however it does not create as big of a spike of cortisol (stress) in your body. Also since you are not working at a maximum effort it is therefore easier to recover from (barring any injuries). 
LSS will help you burn fat over the long run as well, the results are not as quick as HIIT. Studies have found that you lose slightly less with long steady state, but it’s close.

High Intensity Interval Training: 

Performing at 90-100% of your HR max, for 2 minutes of less then a rest interval (3 minutes or less) Repeat this 4-6x

HIIT creates a great anaerobic capacity, to be able to go hard, fast for short bursts. The benefit cardiovascularily is that your heart rate spikes but the more you do this the quicker your heart rate will recover to resting after a bout of intense exercise.
This can be done quick and added to your life fairly easily, however it can be hard on your joints (all out sprints ain't easy), and creates a large spike in cortisol. It most certainly will help you drop fat quicker though

If you only perform HIIT all the time, you won't be able to perform aerobically, meaning if you're a soccer player, you're going to be gassed after a certain point. 
If you only perform LSS you could miss out on the benefits of HIIT, it can help push you over plateau you've been experiencing or help you prepare for a long race because you'll be able to handle portions where you need to push yourself a bit harder. 
If your life is particularly stressful, HIIT may not be a good option for you, because your cortisol levels are already too high you shouldn't be pushing them higher (this is how we get overtraining syndrome, or sometimes can make ourselves sick with training). 
LSS is more time consuming so if you're just trying to add a bit more movement into your life, HIIT might be where it's at. 

So the moral of this story, you probably need both! Both help balance each other out and can be such a beneficial addition to anyone's training. It's not one vs the other, it's finding the appropriate time for both, and seeing where they fit in your training program.