The Myth of the Fat-burning Zone
This doesn't quite answer your question, but I was finally told straight out by a very, very knowledgeable person in the health and fitness industry that you are not necessarily burning more fat when you are in the fat-burning zone. You use the energy stored in fat when you are in the "fat burning zone." The key is (and will always be) expending calories. You expend more calories at a higher heart rate then at a lower heart rate (but a factor is how long can you keep the higher heart rate up). I think, then, if you can keep the heart rate higher during the work out time by breaking the work out into smaller portions (resting in between), you will be burning more calories and will be more successful - unless you have a milk shake during the breaks 
I found a couple articles that address. Below is an excerpt from one and a link to each:
There are two reasons why the claim about a "fat burning zone" is wrong.
First, it does not matter where the fuel comes from while you are exercising; in the end (ie later in the day) your body will replenish and rebalance your energy stores. So a calorie surplus will always end up increasing your body fat; and a calorie deficit will always end up reducing your body fat. The source of fuel while you are exercising is irrelevant.
Second, what matters is the total amount of calories you burn; not the proportion that come fat. If you exercise at lower intensity, a higher proportion may come from body fat, but this may be a smaller absolute number of calories.
In other words, the fat burning zone is a myth caused by misunderstanding the science.
FAQ: Calories to lose weight
The Myth of the Fat-burning Zone
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