calories per gram of protein provided. If you are trying to get 60 g/day of protein from shakes (based on you drinking 3 20 g Muscle Milks (MM) per day), the difference would only be 20 calories more for the Premier Protein (PP), not 180 calories.
60 g protein from MM = 3 shakes and 300 calories
60 g protein from PP = 2 shakes and 320 calories