I’m inclined to agree with @devhammer on the food for fat loss/exercise for fitness concept. However, if you are fat adapted, cardio can do a great deal to push fat loss along. It is harder for a sugar burner as cardio typically makes them hungrier.
To your original question, I would do both. The long, slow distance will keep you in a state where you’re not burning muscle glycogen. And if you do this fasted, you’ll likely become a fat burning furnace. The sprints/HIIT will burn through the glycogen, and your body will have to work to replace it. Some reference this as a rise in metabolism. But I prefer to see it as building your dual fuel capacity.