The best place to start when choosing your level of support is to think about the activities you do. How much bounce do you put your breasts through?
Low Impact: Walking, Yoga, Pilates, Hiking
Medium Impact: Weight Training, Cycling, Martial Arts
High Impact: Running, Aerobics, Court & Field Sports, X Training, High Intensity Cardio
As well as the types of activity it is also important to note that the larger the cup size, the higher the support level is required.
It is also a personal choice of what feels comfortable and the look and style of the sports bra.