You burn more body fat because your body is working on the fat that is already stored since it doesn't have much of anything else to feed on.
You burn the same amount of calories no matter what time of day you do something, but you are more likely to workout for longer periods in the morning because of the lack of food weight and the day hasn't taken it's toll on you, yet. This is where the theory of more calories comes from. You are able to work out longer so you end up burning more calories.
Because of this, I do agree with coolazchic. Aim for mornings. It's also a great way to energize you for the day.