You want some quick carbs before the workout, to get you THROUGH the workout. After the workout (in that 15-30 minute window that was mentioned), you want something high in carbs with a little protein.
The point of the after-workout snack is to stop your body from eating your muscles for energy. The last thing you'd want is for your workout to burn muscle instead of fat. Muscles are dense, so you look slimmer at any given weight, and they take calories to maintain, so they help you lose weight!