Almost any common form of protein is fine. If you want to optimize your protein intake, make sure there is a steady release of amino acids in the blood stream, with more being released during a workout (along with some carbs, IMHO). For instance, you could have a chicken breast and a sweet potato 1-1.5 hours before working out, or whey protein 30 minutes before working out, or both. For post workout nutrition, whey probably works better, but if you already have slowly digesting protein in your body, it doesn't matter so much. Casein has been shown to be a good protein for dieting, as it releases very slowly and may be more muscle sparing because of that.