CLAIM: If you do fasted cardio you're going to use more fat for energy rather than carbohydrates - when you control everything else (intensity of workouts, amount of calories).
ANSWER: It really comes down to what you prefer. There's not going to be a huge difference. There's not enough evidence to support fasted cardio burning more fat in the long term.
Fat loss during fasted cardio is based on two ideas:
1. Lower insulin in a fasted state. Insulin is responsible for suppressing lipolysis. Doing cardio when insulin is low should release that inhibition on lipolysis and allow more fat to be burned during the session.
2. Glycogen (stored carbs) levels are low in the morning. The body should turn to stored fat to supply energy for cardio.
BUT, just because you burn more fat during the cardio session itself, doesn't mean you will lose more fat overall.
Some studies state that whatever you burn more of during fasted cardio (carbs or fat), you will burn less of that during the rest of the next 24 hours (Paoli in 2011). Some of these studies are for short periods of time or small test groups, therefore not the most reliable.
There are studies that show it increases fat oxidation, but there's a difference between that and long term weight loss (NASM).
The best way to go about weight loss and fat loss with exercise is more likely to be enhanced by creating a consistent caloric deficit, rather than exercising in a fasted or fed state. Fasted cardio will not hurt you. There is no special benefit to fasted cardio either. Some people prefer it in the AM and some prefer it in the PM.
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Sources: Paoli, 2011, https://www.ncbi.nlm.nih.gov/pubmed/21411835
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