For best results, before AND after. For the one afterwards you should take some carbohydrate or glutamine too and inject insulin (if you're type 1) accordingly. Additionally, one when you get out of and one before you go to bed will reduce muscle soreness if you're on a moderate-heavy weight training schedule.
Bear in mind that protein CAN have an effect on glucose levels, especially if you're a low-carber. You'll have to learn how to compensate by experience.