To burn fat your body needs to be working at a low intensity during exercise. Both recumbent and upright bikes can achieve this goal. The problem is that although this will allow for a higher percentage of fat being burned during the exercise, very few total calories are burned. To achieve fat loss, you must increase your metabolism so you are burning fat and calories after the workout. To do this, you must perform high-intensity training in addition to the low intensity, steady-state fat burning. The recumbent bike is designed to reduce the stress on your body by offering lower back support. The problem is that the angle of the seat makes it harder to push the heart rate up or to produce a lot of watts. This is fine for your recovery periods, but will slow down your overall caloric expenditure. The upright bike is designed to do both the steady-state exercise as well as short, high-intensity intervals. For overall fat loss, the upright bike will give you more benefits in the long run.
More Answers from National Academy of Sports Medicine