a) I always recommend that you cycle your supplements so that your body does not get used to them thereby decreasing their effect,
b) for variety sakes alone, it breaks the monotony and allows you to try different brands,
c) to take advantage of the unique qualities offered by various brands, and
d) it is a good idea to switch the powders you use on a frequent basis to not develop any food intolerances or even worse allergies (I have an article coming out on this subject in the near future.) A blend that combines various protein sources (i.e. casein, egg, whey, and even beef if you can find it) is your best bet. I highly recommend the following: MD+ Myosin Protein Complex, Beverly International Muscle Provider, Biotest Low-Carb GROW!, and Dorian Yates Approved ProPeptide.
High-quality whey protein powders are excellent choices for post- workout nutrition since they get into your system fast to feed those depleted muscles. Casein, on the other hand, is a slow releasing protein as it recurdles into a solid in your gut prolonging digestion - this would be an ideal choice prior to your workouts or before going to bed. Anyhow, as far as whey powders are concerned, amino acid complex profile determines quality. I have in my possession a list of protein powders that were analyzed for quality by an independent laboratory, but since I don't want any of these companies breathing down my neck, I will only divulge that information to my clients. Remember one thing, you get what you pay for! Keep that in mind. Also, some people are quite sensitive to aspartame and lactose so you will have to find free versions of those. Taste will ultimately determine whether you purchase that powder again. Write in with some of your feedback on these products - I'd love to hear your comments.
One more thing, you could even mix protein powder into other foods to up the protein content and improve the flavor. Mixing protein powder in (slow-cooked) oatmeal is one option, but here's another. I call it my "bedtime concoction." Add a scoop of Xtreme Formulations Ultra Peptide (vanilla) to a mixture of ricotta (whey) and cottage (casein) cheese. Not only does this provide both an anabolic and anticatabolic effect during sleep, it also tastes amazing - it's like eating the cream filling of a cannoli without the pastry shell! Try it.